dt-bindings: sunxi: Add Allwinner A80 PRCM Binding

Even though we've used the A80 PRCM driver for some time and a number of
boards using it already, we never had a binding for it. Let's add it
based on what the driver expects and the boards are providing.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-36-maxime@cerno.tech

Documentation/devicetree/bindings/arm/sunxi/allwinner,sun9i-a80-prcm.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/sunxi/allwinner,sun9i-a80-prcm.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Allwinner A80 PRCM Device Tree Bindings
+
+maintainers:
+  - Chen-Yu Tsai <wens@csie.org>
+  - Maxime Ripard <mripard@kernel.org>
+
+properties:
+  compatible:
+    const: allwinner,sun9i-a80-prcm
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+      prcm@8001400 {
+          compatible = "allwinner,sun9i-a80-prcm";
+          reg = <0x08001400 0x200>;
+      };
+
+...