dt-bindings: clock: add mtmips SoCs system controller

Adds device tree binding documentation for system controller node present
in Mediatek MIPS and Ralink SOCs. This node is a clock and reset provider
for the rest of the world. This covers RT2880, RT3050, RT3052, RT3350,
RT3883, RT5350, MT7620, MT7628 and MT7688 SoCs.
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Documentation/devicetree/bindings/clock/mediatek,mtmips-sysc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/mediatek,mtmips-sysc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MTMIPS SoCs System Controller
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+description: |
+  MediaTek MIPS and Ralink SoCs provides a system controller to allow
+  to access to system control registers. These registers include clock
+  and reset related ones so this node is both clock and reset provider
+  for the rest of the world.
+
+  These SoCs have an XTAL from where the cpu clock is
+  provided as well as derived clocks for the bus and the peripherals.
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - ralink,mt7620-sysc
+          - ralink,mt7628-sysc
+          - ralink,mt7688-sysc
+          - ralink,rt2880-sysc
+          - ralink,rt3050-sysc
+          - ralink,rt3052-sysc
+          - ralink,rt3352-sysc
+          - ralink,rt3883-sysc
+          - ralink,rt5350-sysc
+      - const: syscon
+
+  reg:
+    maxItems: 1
+
+  '#clock-cells':
+    description:
+      The first cell indicates the clock number.
+    const: 1
+
+  '#reset-cells':
+    description:
+      The first cell indicates the reset bit within the register.
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - '#clock-cells'
+  - '#reset-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    syscon@0 {
+      compatible = "ralink,rt5350-sysc", "syscon";
+      reg = <0x0 0x100>;
+      #clock-cells = <1>;
+      #reset-cells = <1>;
+    };