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/behaviors/soft_off.dtsi

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/ {
+    behaviors {
+        /omit-if-no-ref/ soft_off: behavior_soft_off {
+            compatible = "zmk,behavior-soft-off";
+            label = "SOFTOFF";
+            #binding-cells = <0>;
+        };
+    };
+};