dt-bindings: phy: Add lan966x-serdes binding

Document the lan966x ethernet serdes phy driver bindings.
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NHoratiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20211116100818.1615762-2-horatiu.vultur@microchip.comSigned-off-by: NVinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/microchip,lan966x-serdes.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/microchip,lan966x-serdes.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Microchip Lan966x Serdes controller
+
+maintainers:
+  - Horatiu Vultur <horatiu.vultur@microchip.com>
+
+description: |
+  Lan966x has 7 interfaces, consisting of 2 copper transceivers(CU),
+  3 SERDES6G and 2 RGMII interfaces. Two of the SERDES6G support QSGMII.
+  Also it has 8 logical Ethernet ports which can be connected to these
+  interfaces. The Serdes controller will allow to configure these interfaces
+  and allows to "mux" the interfaces to different ports.
+
+  For simple selection of the interface that is used with a port, the
+  following macros are defined CU(X), SERDES6G(X), RGMII(X). Where X is a
+  number that represents the index of that interface type. For example
+  CU(1) means use interface copper transceivers 1. SERDES6G(2) means use
+  interface SerDes 2.
+
+properties:
+  $nodename:
+    pattern: "^serdes@[0-9a-f]+$"
+
+  compatible:
+    const: microchip,lan966x-serdes
+
+  reg:
+    items:
+      - description: HSIO registers
+      - description: HW_STAT register
+
+  '#phy-cells':
+    const: 2
+    description: |
+      - Input port to use for a given macro.
+      - The macro to be used. The macros are defined in
+        dt-bindings/phy/phy-lan966x-serdes.
+
+required:
+  - compatible
+  - reg
+  - '#phy-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    serdes: serdes@e2004010 {
+      compatible = "microchip,lan966x-serdes";
+      reg = <0xe202c000 0x9c>, <0xe2004010 0x4>;
+      #phy-cells = <2>;
+    };
+
+...