feat(power): Add power domain config

* High level ZMK_POWER_DOMAINS for the feature.
* ZMK_POWER_DOMAINS_DYNAMIC_DEFAULT config setting
   to have various subsystems add their respective devices to
   the chosen `zmk,default-power-domain` on init.

app/Kconfig

@@ -273,9 +273,16 @@ if ZMK_RGB_UNDERGLOW
 config SPI
     default y
 
+
+config ZMK_RGB_UNDERGLOW_DEFAULT_POWER_DOMAIN
+    bool "Auto-assign the RGB underglow to the default power domain"
+    default y
+    depends on ZMK_POWER_DOMAINS_DYNAMIC_DEFAULT
+
 config ZMK_RGB_UNDERGLOW_EXT_POWER
     bool "RGB underglow toggling also controls external power"
-    default y if !POWER_DOMAIN
+    default y
+    depends on !ZMK_POWER_DOMAINS
 
 config ZMK_RGB_UNDERGLOW_BRT_MIN
     int "RGB underglow minimum brightness in percent"
@@ -338,6 +345,7 @@ config ZMK_RGB_UNDERGLOW_AUTO_OFF_USB
     bool "Turn off RGB underglow when USB is disconnected"
     depends on USB_DEVICE_STACK
 
+
 #ZMK_RGB_UNDERGLOW
 endif
 
@@ -412,23 +420,36 @@ config ZMK_SLEEP
     depends on HAS_POWEROFF
     select POWEROFF
     select ZMK_PM_DEVICE_SUSPEND_RESUME
+    select PM_DEVICE
     imply USB
 
-if ZMK_SLEEP
-
-config PM_DEVICE
-    default y
-
 config ZMK_IDLE_SLEEP_TIMEOUT
     int "Milliseconds of inactivity before entering deep sleep"
+    depends on ZMK_SLEEP
     default 900000
 
-#ZMK_SLEEP
-endif
+config ZMK_POWER_DOMAINS
+    bool "Enable automatic power domain handling"
+    default n
+    select PM_DEVICE
+    select PM_DEVICE_POWER_DOMAIN
+    select PM_DEVICE_RUNTIME
+    select POWER_DOMAIN
+    select POWER_DOMAIN_GPIO
+
+config ZMK_POWER_DOMAINS_DYNAMIC_DEFAULT
+    bool "Enable assigning peripherals to the chosen default power domain"
+    default n
+    depends on ZMK_POWER_DOMAINS
+    select PM_DEVICE_POWER_DOMAIN_DYNAMIC
+
+# Default this to 2, the most common scenario for this for ZMK is RGB + OLED
+config PM_DEVICE_POWER_DOMAIN_DYNAMIC_NUM
+    default 2
 
 config ZMK_EXT_POWER
     bool "Enable support to control external power output"
-    default y
+    default y if !ZMK_POWER_DOMAINS
 
 config ZMK_PM
     bool

app/src/display/Kconfig

@@ -13,6 +13,11 @@ config ZMK_DISPLAY_BLANK_ON_IDLE
     bool "Blank display on idle"
     default y if SSD1306
 
+config ZMK_DISPLAY_DEFAULT_POWER_DOMAIN
+    bool "Auto-assign the display to the default power domain"
+    default y
+    depends on ZMK_POWER_DOMAINS_DYNAMIC_DEFAULT
+
 if LV_USE_THEME_MONO
 
 config ZMK_DISPLAY_INVERT

app/src/display/main.c

@@ -7,6 +7,7 @@
 #include <zephyr/kernel.h>
 #include <zephyr/init.h>
 #include <zephyr/device.h>
+#include <zephyr/pm/device.h>
 #include <zephyr/pm/device_runtime.h>
 #include <zephyr/devicetree.h>
 
@@ -118,6 +119,19 @@ void initialize_display(struct k_work *work) {
         return;
     }
 
+#if IS_ENABLED(CONFIG_ZMK_DISPLAY_DEFAULT_POWER_DOMAIN) && DT_HAS_CHOSEN(zmk_default_power_domain)
+
+    pm_device_runtime_enable(display);
+    if (!pm_device_on_power_domain(display)) {
+        int rc =
+            pm_device_power_domain_add(display, DEVICE_DT_GET(DT_CHOSEN(zmk_default_power_domain)));
+        if (rc < 0) {
+            LOG_ERR("Failed to add the display to the default power domain (0x%02x)", -rc);
+        }
+    }
+
+#endif
+
     initialized = true;
 
     initialize_theme();

app/src/rgb_underglow.c

@@ -7,6 +7,7 @@
 #include <zephyr/device.h>
 #include <zephyr/init.h>
 #include <zephyr/kernel.h>
+#include <zephyr/pm/device.h>
 #include <zephyr/pm/device_runtime.h>
 #include <zephyr/settings/settings.h>
 
@@ -254,6 +255,20 @@ static int zmk_rgb_underglow_init(void) {
     }
 #endif
 
+#if IS_ENABLED(CONFIG_ZMK_RGB_UNDERGLOW_DEFAULT_POWER_DOMAIN) &&                                   \
+    DT_HAS_CHOSEN(zmk_default_power_domain)
+
+    pm_device_runtime_enable(led_strip);
+    if (!pm_device_on_power_domain(led_strip)) {
+        int rc = pm_device_power_domain_add(led_strip,
+                                            DEVICE_DT_GET(DT_CHOSEN(zmk_default_power_domain)));
+        if (rc < 0) {
+            LOG_ERR("Failed to add the LED strip to the default power domain (0x%02x)", -rc);
+        }
+    }
+
+#endif
+
     state = (struct rgb_underglow_state){
         color : {
             h : CONFIG_ZMK_RGB_UNDERGLOW_HUE_START,