feat: Add soft on/off support.

Initial work on a soft on/off support for ZMK. Triggering soft off
puts the device into deep sleep with only a specific GPIO pin
configured to wake the device, avoiding waking from other key
presses in the matrix like the normal deep sleep.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/dts/bindings/zmk,wakeup-trigger-key.yaml

@@ -0,0 +1,18 @@
+# Copyright (c) 2023 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  Driver for a dedicated key for waking the device from sleep
+
+compatible: "zmk,wakeup-trigger-key"
+
+include: base.yaml
+
+properties:
+  trigger:
+    type: phandle
+    required: true
+    description: The GPIO key that triggers wake via interrupt
+  extra-gpios:
+    type: phandle-array
+    description: Optional set of pins that should be set active before sleeping.