refactor(ext_power): Remove label property

Changed the label property on zmk,ext-power-generic to be optional and
removed it from existing uses. Renamed the nodes for all non-development
boards to "EXT_POWER" to preserve user settings.

rgb_underglow.c now finds the correct device by finding the first
instance of zmk,ext-power-generic instead of looking for a node named
"EXT_POWER".

app/dts/bindings/zmk,ext-power-generic.yaml

@@ -14,7 +14,8 @@ properties:
     required: true
   label:
     type: string
-    required: true
+    required: false
+    deprecated: true
   init-delay-ms:
     type: int
     description: Number of milliseconds to delay after initializing driver