feat(shields): M60 keyboard use back button as power switch

app/boards/shields/m60/CMakeLists.txt

@@ -0,0 +1,3 @@
+zephyr_library()
+zephyr_library_sources(pinmux.c)
+zephyr_library_include_directories(${ZEPHYR_BASE}/drivers)

app/boards/shields/m60/pinmux.c

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+#include <zephyr/init.h>
+#include <zephyr/drivers/gpio.h>
+#include <zephyr/sys/sys_io.h>
+#include <zephyr/devicetree.h>
+
+/* The PMU(BQ24075) enable pin(LOW active) on M60 is controlled by a NAND gate.
+ * Partial reverse engineering shows:
+ * P0.28 affects the NAND gate.
+ * The button affects P0.27 the NAND gate.
+ * P0.03 is detection pin for charging state(possibly attached to PMU LED pin).
+ */
+
+static const struct device *p0 = DEVICE_DT_GET(DT_NODELABEL(gpio0));
+
+static inline void power_off(void) {
+    gpio_pin_set(p0, 28, 0); // turn off the PMU battery path
+}
+
+static inline bool is_charging(void) {
+    // 0: charging
+    return gpio_pin_get_raw(p0, 3) == 0;
+}
+
+static void button_pressed(const struct device *dev, struct gpio_callback *cb, uint32_t pins) {
+    // wait the button stable at 0 so the keyboard won't be powered up accidentally
+    k_sleep(K_SECONDS(1));
+
+    if (!is_charging()) {
+        power_off();
+    }
+}
+
+static int pinmux_nrf52840_m2_init(void) {
+    // back button
+    // NOTE: if used to power off the keyboard, make sure the action is done
+    // AFTER the button is released.
+    //
+    // To wake up the keyboard from sleep with this button, must use interrupt.
+    // To avoid the callback triggered after keyboard woke up(powering off the
+    // keyboard), must use GPIO_INT_EDGE_FALLING(trigger on button pressed).
+    gpio_pin_interrupt_configure(p0, 27, GPIO_INT_EDGE_FALLING);
+
+    // GPIO 0.28(LDO control) is already configured by bootloader,
+    // so configuring it here is just an ensurance.
+    gpio_pin_configure(p0, 28, GPIO_OUTPUT_ACTIVE | GPIO_PULL_UP | GPIO_OPEN_DRAIN);
+
+    // this is the pin for charging detection
+    gpio_pin_configure(p0, 3, GPIO_INPUT | GPIO_PULL_UP);
+
+    // setup the interrupt handler to poweroff the keyboard
+    static struct gpio_callback button_cb;
+    gpio_init_callback(&button_cb, button_pressed, BIT(27));
+    gpio_add_callback(p0, &button_cb);
+
+    return 0;
+}
+
+SYS_INIT(pinmux_nrf52840_m2_init, APPLICATION, CONFIG_APPLICATION_INIT_PRIORITY);