dt-bindings: arm: mediatek: sgmiisys: Convert to DT schema

Convert mediatek,sgmiiisys bindings to DT schema format.
Add maintainer Matthias Brugger, no maintainers were listed in the
original documentation.
As this node is also referenced by the Ethernet controller and used
as SGMII PCS add this fact to the description.
Move the file to Documentation/devicetree/bindings/net/pcs/ which seems
more appropriate given that the great majority of registers are related
to SGMII PCS functionality and only one register represents clock bits.

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Daniel Golle <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/arm/mediatek/mediatek,sgmiisys.txt

···
-MediaTek SGMIISYS controller
-============================
-
-The MediaTek SGMIISYS controller provides various clocks to the system.
-
-Required Properties:
-
-- compatible: Should be:
-	- "mediatek,mt7622-sgmiisys", "syscon"
-	- "mediatek,mt7629-sgmiisys", "syscon"
-	- "mediatek,mt7981-sgmiisys_0", "syscon"
-	- "mediatek,mt7981-sgmiisys_1", "syscon"
-	- "mediatek,mt7986-sgmiisys_0", "syscon"
-	- "mediatek,mt7986-sgmiisys_1", "syscon"
-- #clock-cells: Must be 1
-
-The SGMIISYS controller uses the common clk binding from
-Documentation/devicetree/bindings/clock/clock-bindings.txt
-The available clocks are defined in dt-bindings/clock/mt*-clk.h.
-
-Example:
-
-sgmiisys: sgmiisys@1b128000 {
-	compatible = "mediatek,mt7622-sgmiisys", "syscon";
-	reg = <0 0x1b128000 0 0x1000>;
-	#clock-cells = <1>;
-};

Documentation/devicetree/bindings/net/pcs/mediatek,sgmiisys.yaml

···
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/pcs/mediatek,sgmiisys.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek SGMIISYS Controller
+
+maintainers:
+  - Matthias Brugger <[e-mail]>
+
+description:
+  The MediaTek SGMIISYS controller provides a SGMII PCS and some clocks
+  to the ethernet subsystem to which it is attached.
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - mediatek,mt7622-sgmiisys
+          - mediatek,mt7629-sgmiisys
+          - mediatek,mt7986-sgmiisys_0
+          - mediatek,mt7986-sgmiisys_1
+      - const: syscon
+
+  reg:
+    maxItems: 1
+
+  '#clock-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - '#clock-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    soc {
+      #address-cells = <2>;
+      #size-cells = <2>;
+      sgmiisys: syscon@1b128000 {
+        compatible = "mediatek,mt7622-sgmiisys", "syscon";
+        reg = <0 0x1b128000 0 0x1000>;
+        #clock-cells = <1>;
+      };
+    };