dt-bindings: mfd: syscon: Split and enforce documenting MFD children

Simple syscon nodes can be documented in common syscon.yaml, however
devices with simple-mfd compatible, thus with some children, should have
their own schema listing these children.  Such listing makes the binding
specific, allows better validation (so the incorrect child would not
appear in the simple-mfd node) and actually enforces repeated rule for
simple-mfd devices:

  "simple-mfd" is only for simple devices, where the children do not
  depend on the parent.

Currently the syscon+simple-mfd binding is quite broad and allows
any child or property, thus above rule cannot be enforced.

Split the syscon.yaml binding into:
1. Common syscon properties, used potentially by many bindings.
2. Simple syscon devices (NO simple-mfd!).
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240627-dt-bindings-mfd-syscon-split-v4-6-dc6699a9f3e4@linaro.orgSigned-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/mfd/syscon-common.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/syscon-common.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: System Controller Registers R/W Common Properties
+
+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@kernel.org>
+
+select:
+  properties:
+    compatible:
+      contains:
+        const: syscon
+
+  required:
+    - compatible
+
+properties:
+  compatible:
+    contains:
+      const: syscon
+    minItems: 2
+    maxItems: 5  # Should be enough
+
+  reg:
+    maxItems: 1
+
+  reg-io-width:
+    description:
+      The size (in bytes) of the IO accesses that should be performed
+      on the device.
+    enum: [1, 2, 4, 8]
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: simple-mfd
+    then:
+      properties:
+        compatible:
+          minItems: 3
+          maxItems: 5
+
+additionalProperties: true
+
+examples:
+  - |
+    syscon: syscon@1c00000 {
+        compatible = "allwinner,sun8i-h3-system-controller", "syscon";
+        reg = <0x01c00000 0x1000>;
+    };
+...