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/boards/arm/pillbug/pillbug.dts

@@ -30,9 +30,9 @@
         };
     };
 
-    ext-power {
+    // Node name must match original "EXT_POWER" label to preserve user settings.
+    EXT_POWER {
         compatible = "zmk,ext-power-generic";
-        label = "EXT_POWER";
         control-gpios = <&gpio1 7 GPIO_ACTIVE_LOW>;
         init-delay-ms = <50>;
     };