dt: bindings: add mt7621-sysc device tree binding documentation

Adds device tree binding documentation for clocks in the
MT7621 SOC.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210309052226.29531-3-sergio.paracuellos@gmail.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/mediatek,mt7621-sysc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/mediatek,mt7621-sysc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MT7621 Clock Device Tree Bindings
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+description: |
+  The MT7621 has a PLL controller from where the cpu clock is provided
+  as well as derived clocks for the bus and the peripherals. It also
+  can gate SoC device clocks.
+
+  Each clock is assigned an identifier and client nodes use this identifier
+  to specify the clock which they consume.
+
+  All these identifiers could be found in:
+  [1]: <include/dt-bindings/clock/mt7621-clk.h>.
+
+  The clocks are provided inside a system controller node.
+
+properties:
+  compatible:
+    items:
+      - const: mediatek,mt7621-sysc
+      - const: syscon
+
+  reg:
+    maxItems: 1
+
+  "#clock-cells":
+    description:
+      The first cell indicates the clock number, see [1] for available
+      clocks.
+    const: 1
+
+  ralink,memctl:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      phandle of syscon used to control memory registers
+
+  clock-output-names:
+    maxItems: 8
+
+required:
+  - compatible
+  - reg
+  - '#clock-cells'
+  - ralink,memctl
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/mt7621-clk.h>
+
+    sysc: sysc@0 {
+      compatible = "mediatek,mt7621-sysc", "syscon";
+      reg = <0x0 0x100>;
+      #clock-cells = <1>;
+      ralink,memctl = <&memc>;
+      clock-output-names = "xtal", "cpu", "bus",
+                           "50m", "125m", "150m",
+                           "250m", "270m";
+    };