dt-bindings: arm,versatile-sysreg: Convert to DT schema format

Convert the arm,versatile-sysreg binding to DT schema format.

The original binding was missing 'simple-mfd' and a 'panel' sub node which
the only user (versatile-ab.dts) of this binding has.
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220810160341.51995-3-robh@kernel.org

Documentation/devicetree/bindings/arm/arm,versatile-sysreg.yaml

+# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/arm,versatile-sysreg.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Arm Versatile system registers
+
+maintainers:
+  - Linus Walleij <linus.walleij@linaro.org>
+
+description:
+  This is a system control registers block, providing multiple low level
+  platform functions like board detection and identification, software
+  interrupt generation, MMC and NOR Flash control, etc.
+
+properties:
+  compatible:
+    items:
+      - const: arm,versatile-sysreg
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+  panel:
+    type: object
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+...

Documentation/devicetree/bindings/arm/versatile-sysreg.txt

-ARM Versatile system registers
---------------------------------------
-
-This is a system control registers block, providing multiple low level
-platform functions like board detection and identification, software
-interrupt generation, MMC and NOR Flash control etc.
-
-Required node properties:
-- compatible value : = "arm,versatile-sysreg", "syscon"
-- reg : physical base address and the size of the registers window