dt-bindings: phy: describe the Qualcomm SGMII PHY

Describe the SGMII/SerDes PHY present on the sa8775p platforms.
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230619091336.194914-3-brgl@bgdev.plSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/qcom,sa8775p-dwmac-sgmii-phy.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/qcom,sa8775p-dwmac-sgmii-phy.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm SerDes/SGMII ethernet PHY controller
+
+maintainers:
+  - Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
+
+description:
+  The SerDes PHY sits between the MAC and the external PHY and provides
+  separate Rx Tx lines.
+
+properties:
+  compatible:
+    const: qcom,sa8775p-dwmac-sgmii-phy
+
+  reg:
+    items:
+      - description: serdes
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    const: sgmi_ref
+
+  phy-supply:
+    description:
+      Phandle to a regulator that provides power to the PHY.
+
+  "#phy-cells":
+    const: 0
+
+required:
+  - compatible
+  - reg
+  - "#phy-cells"
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/qcom,sa8775p-gcc.h>
+    serdes_phy: phy@8901000 {
+        compatible = "qcom,sa8775p-dwmac-sgmii-phy";
+        reg = <0x08901000 0xe10>;
+        clocks = <&gcc GCC_SGMI_CLKREF_EN>;
+        clock-names = "sgmi_ref";
+        #phy-cells = <0>;
+    };