dt-bindings: phy: Add binding for Mediatek MT7621 PCIe PHY

Add bindings to describe Mediatek MT7621 PCIe PHY.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201121155037.21354-2-sergio.paracuellos@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/mediatek,mt7621-pci-phy.yaml

+# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/phy/mediatek,mt7621-pci-phy.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Mediatek Mt7621 PCIe PHY Device Tree Bindings
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+properties:
+  compatible:
+    const: mediatek,mt7621-pci-phy
+
+  reg:
+    maxItems: 1
+
+  "#phy-cells":
+    const: 1
+    description: selects if the phy is dual-ported
+
+required:
+  - compatible
+  - reg
+  - "#phy-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    pcie0_phy: pcie-phy@1e149000 {
+      compatible = "mediatek,mt7621-pci-phy";
+      reg = <0x1e149000 0x0700>;
+      #phy-cells = <1>;
+    };