devicetree: add properties for power supply control

Provide a common set of properties for various ways of controlling
power:
* supply-gpios for a GPIO specifier acting like a switch
* vin-supply for a reference to a regulator device

Document the behavior expected when these properties are present.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

dts/bindings/base/power.yaml

@@ -0,0 +1,33 @@
+# Copyright (c) 2020, Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+# Properties for nodes with controllable power supplies.
+
+properties:
+    supply-gpios:
+      type: phandle
+      required: false
+      description: |
+        GPIO specifier that controls power to the device.
+
+        This property should be provided when the device has a dedicated
+        switch that controls power to the device.  The supply state is
+        entirely the responsibility of the device driver.
+
+        Contrast with vin-supply.
+
+    vin-supply:
+      type: phandle
+      required: false
+      description: |
+        Reference to the regulator that controls power to the device.
+        The referenced devicetree node must have a regulator compatible.
+
+        This property should be provided when device power is supplied
+        by a shared regulator.  The supply state is dependent on the
+        request status of all devices fed by the regulator.
+
+        Contrast with supply-gpios.  If both properties are provided
+        then the regulator must be requested before the supply GPIOS is
+        set to an active state, and the supply GPIOS must be set to an
+        inactive state before releasing the regulator.