Added driver to control the external power output

This PR adds support to control the external power output from controllers like nice!nano, nRFMicro etc

I have implemented based on my understanding of Pete suggestion on this feature.

Testing done:

    Tested by enabling and disabling the ext_power from application and verified
    Verified the application does not crash with boards that does not have ext_power support
    Note:
    I did not test this in nice!nano since I don't have the boards. Will get help from others once the behavior PR is up

Next Steps:

    Create a behavior PR to control enable/disable ext_power

app/dts/bindings/behaviors/zmk,behavior-sensor-rotate-key-press.yaml

@@ -1,4 +1,4 @@
-# Copyright (c) 2020, Pete Johanson
+# Copyright (c) 2020, The ZMK Contributors
 # SPDX-License-Identifier: MIT
 
 description: Sensor rotate key press/release behavior

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

@@ -0,0 +1,20 @@
+#
+# Copyright (c) 2020, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+#
+
+description: |
+  Generic driver for controlling the external power output
+  by toggling the control-gpio pin status
+  (Only in supported hardware)
+
+compatible: "zmk,ext-power-generic"
+
+properties:
+  control-gpios:
+    type: phandle-array
+    required: true
+  label:
+    type: string
+    required: true
+

app/dts/bindings/zmk,keymap-sensors.yaml

@@ -1,3 +1,8 @@
+#
+# Copyright (c) 2020, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+#
+
 description: |
   Allows defining the collection of sensors bound in the keymap layers