refactor: Move away from deprecated label usages.

* 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.

app/drivers/zephyr/dts/bindings/sensor/zmk,battery-nrf-vddh.yaml

@@ -4,8 +4,3 @@
 description: Battery SoC monitoring using nRF VDDH
 
 compatible: "zmk,battery-nrf-vddh"
-
-properties:
-  label:
-    required: true
-    type: string

app/drivers/zephyr/dts/bindings/sensor/zmk,battery-voltage-divider.yaml

@@ -6,8 +6,3 @@ description: Battery SoC monitoring using voltage divider
 compatible: "zmk,battery-voltage-divider"
 
 include: voltage-divider.yaml
-
-properties:
-  label:
-    required: true
-    type: string