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/src/kscan.c

@@ -6,6 +6,7 @@
 
 #include <zephyr/kernel.h>
 #include <zephyr/device.h>
+#include <zephyr/pm/device.h>
 #include <zephyr/bluetooth/addr.h>
 #include <zephyr/drivers/kscan.h>
 #include <zephyr/logging/log.h>
@@ -75,6 +76,11 @@ int zmk_kscan_init(const struct device *dev) {
 
     kscan_config(dev, zmk_kscan_callback);
     kscan_enable_callback(dev);
+#if IS_ENABLED(CONFIG_PM_DEVICE)
+    if (pm_device_wakeup_is_capable(dev)) {
+        pm_device_wakeup_enable(dev, true);
+    }
+#endif // IS_ENABLED(CONFIG_PM_DEVICE)
 
     return 0;
 }