With ADC pin, maximum 3.6V input needs gain 1/6 to be less than the 0.6V
reference.
With VDDHDIV5, maximum 6V input corresponds to 1.2V so gain 1/2 will be less
than the 0.6V reference and be slightly more precise.
* Move away from DT_LABEL.
* Move to DEVICE_DT_GET for non-behavior device access.
* Move various drivers to `gpio_spec_dt` and `DT` related macros.
* Remove mcp23017 while at it, since better upstream driver is
available.
CONFIG_ZMK_BATTERY_NRF_VDDH and CONFIG_ZMK_BATTERY_VOLTAGE_DIVIDER are
now automatically enabled if a DT node with the matching "compatible"
property is present and enabled, so they no longer need to be manually
set for each board.
