dt-bindings: arm: convert vexpress-sysregs to DT schema

The Arm Versatile Express system control register block provides GPIO
functionality to some devices and is also used for board identification.

Extract the first half of the informal vexpress-sysreg.txt binding and
make it proper DT schema compliant.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-7-andre.przywara@arm.com

Documentation/devicetree/bindings/arm/vexpress-sysreg.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/vexpress-sysreg.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM Versatile Express system registers bindings
+
+maintainers:
+  - Andre Przywara <andre.przywara@arm.com>
+
+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:
+    const: arm,vexpress-sysreg
+
+  reg:
+    maxItems: 1
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 1
+
+  ranges: true
+
+additionalProperties: false
+
+patternProperties:
+  '^gpio@[0-9a-f]+$':
+    type: object
+    additionalProperties: false
+    description:
+      GPIO children
+
+    properties:
+      compatible:
+        enum:
+          - arm,vexpress-sysreg,sys_led
+          - arm,vexpress-sysreg,sys_mci
+          - arm,vexpress-sysreg,sys_flash
+
+      gpio-controller: true
+
+      "#gpio-cells":
+        const: 2
+        description: |
+          The first cell is the function number:
+          for sys_led : 0..7 = LED 0..7
+          for sys_mci : 0 = MMC CARDIN, 1 = MMC WPROT
+          for sys_flash : 0 = NOR FLASH WPn
+          The second cell can take standard GPIO flags.
+
+      reg:
+        maxItems: 1
+
+    required:
+      - compatible
+      - reg
+      - gpio-controller
+      - "#gpio-cells"
+
+required:
+  - compatible
+  - "#address-cells"
+  - "#size-cells"
+
+examples:
+  - |
+    sysreg@0 {
+        compatible = "arm,vexpress-sysreg";
+        reg = <0x00000 0x1000>;
+        #address-cells = <1>;
+        #size-cells = <1>;
+        ranges = <0 0 0x1000>;
+
+        v2m_led_gpios: gpio@8 {
+            compatible = "arm,vexpress-sysreg,sys_led";
+            reg = <0x008 4>;
+            gpio-controller;
+            #gpio-cells = <2>;
+        };
+    };
+
+...