dt-bindings: Add syscon YAML description

The syscon binding is a pretty loose one, with everyone having a bunch of
vendor specific compatibles.

In order to start the effort to describe them using YAML, let's create a
binding that tolerates additional, not listed, compatibles.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mfd/syscon.txt

-* System Controller Registers R/W driver
-
-System controller node represents a register region containing a set
-of miscellaneous registers. The registers are not cohesive enough to
-represent as any specific type of device. The typical use-case is for
-some other node's driver, or platform-specific code, to acquire a
-reference to the syscon node (e.g. by phandle, node path, or search
-using a specific compatible value), interrogate the node (or associated
-OS driver) to determine the location of the registers, and access the
-registers directly.
-
-Required properties:
-- compatible: Should contain "syscon".
-- reg: the register region can be accessed from syscon
-
-Optional property:
-- reg-io-width: the size (in bytes) of the IO accesses that should be
-  performed on the device.
-- hwlocks: reference to a phandle of a hardware spinlock provider node.
-
-Examples:
-gpr: iomuxc-gpr@20e0000 {
-	compatible = "fsl,imx6q-iomuxc-gpr", "syscon";
-	reg = <0x020e0000 0x38>;
-	hwlocks = <&hwlock1 1>;
-};
-
-hwlock1: hwspinlock@40500000 {
-	...
-	reg = <0x40500000 0x1000>;
-	#hwlock-cells = <1>;
-};

Documentation/devicetree/bindings/mfd/syscon.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/syscon.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: System Controller Registers R/W Device Tree Bindings
+
+description: |
+  System controller node represents a register region containing a set
+  of miscellaneous registers. The registers are not cohesive enough to
+  represent as any specific type of device. The typical use-case is
+  for some other node's driver, or platform-specific code, to acquire
+  a reference to the syscon node (e.g. by phandle, node path, or
+  search using a specific compatible value), interrogate the node (or
+  associated OS driver) to determine the location of the registers,
+  and access the registers directly.
+
+maintainers:
+  - Lee Jones <lee.jones@linaro.org>
+
+select:
+  properties:
+    compatible:
+      contains:
+        enum:
+          - syscon
+
+  required:
+    - compatible
+
+properties:
+  compatible:
+    anyOf:
+      - items:
+        - enum:
+          - allwinner,sun8i-a83t-system-controller
+          - allwinner,sun8i-h3-system-controller
+          - allwinner,sun8i-v3s-system-controller
+          - allwinner,sun50i-a64-system-controller
+
+        - const: syscon
+
+      - contains:
+          const: syscon
+        additionalItems: true
+
+  reg:
+    maxItems: 1
+
+  reg-io-width:
+    description: |
+      The size (in bytes) of the IO accesses that should be performed
+      on the device.
+    allOf:
+      - $ref: /schemas/types.yaml#/definitions/uint32
+      - enum: [ 1, 2, 4, 8 ]
+
+  hwlocks:
+    maxItems: 1
+    description:
+      Reference to a phandle of a hardware spinlock provider node.
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    syscon: syscon@1c00000 {
+        compatible = "allwinner,sun8i-h3-system-controller", "syscon";
+        reg = <0x01c00000 0x1000>;
+    };
+
+  - |
+    gpr: iomuxc-gpr@20e0000 {
+        compatible = "fsl,imx6q-iomuxc-gpr", "syscon";
+        reg = <0x020e0000 0x38>;
+        hwlocks = <&hwlock1 1>;
+    };
+
+...

Documentation/devicetree/bindings/misc/allwinner,syscon.txt

-* Allwinner sun8i system controller
-
-This file describes the bindings for the system controller present in
-Allwinner SoC H3, A83T and A64.
-The principal function of this syscon is to control EMAC PHY choice and
-config.
-
-Required properties for the system controller:
-- reg: address and length of the register for the device.
-- compatible: should be "syscon" and one of the following string:
-		"allwinner,sun8i-h3-system-controller"
-		"allwinner,sun8i-v3s-system-controller"
-		"allwinner,sun50i-a64-system-controller"
-		"allwinner,sun8i-a83t-system-controller"
-
-Example:
-syscon: syscon@1c00000 {
-	compatible = "allwinner,sun8i-h3-system-controller", "syscon";
-	reg = <0x01c00000 0x1000>;
-};