Documentation: DT: binding documentation for regulator-poweroff

Add devicetree binding documentation for regulator-poweroff driver.
Signed-off-by: Michael Klein <michael@fossekall.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/reset/regulator-poweroff.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/power/reset/regulator-poweroff.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Force-disable power regulator to turn the power off.
+
+maintainers:
+  - Michael Klein <michael@fossekall.de>
+
+description: |
+  When the power-off handler is called, a power regulator is disabled by
+  calling regulator_force_disable(). If the power is still on and the
+  CPU still running after a 3000ms delay, a warning is emitted.
+
+properties:
+  compatible:
+    const: "regulator-poweroff"
+
+  cpu-supply:
+    description:
+      regulator to disable on power-down
+
+required:
+  - compatible
+  - cpu-supply
+
+additionalProperties: false
+
+examples:
+  - |
+    regulator-poweroff {
+        compatible = "regulator-poweroff";
+        cpu-supply = <&reg_vcc1v2>;
+    };
+...