dt-bindings: phy: mediatek,mt7988-xfi-tphy: add new bindings

Add bindings for the MediaTek XFI Ethernet SerDes T-PHY found in the
MediaTek MT7988 SoC which can operate at various interfaces modes:

via USXGMII PCS:
 * USXGMII
 * 10GBase-R
 * 5GBase-R

via LynxI SGMII PCS:
 * 2500Base-X
 * 1000Base-X
 * Cisco SGMII (MAC side)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/da5498096f71a40ca1eac4124b7bb601c82396fb.1712625857.git.daniel@makrotopia.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/mediatek,mt7988-xfi-tphy.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/mediatek,mt7988-xfi-tphy.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek MT7988 XFI T-PHY
+
+maintainers:
+  - Daniel Golle <daniel@makrotopia.org>
+
+description:
+  The MediaTek XFI SerDes T-PHY provides the physical SerDes lanes
+  used by the (10G/5G) USXGMII PCS and (1G/2.5G) LynxI PCS found in
+  MediaTek's 10G-capabale MT7988 SoC.
+  In MediaTek's SDK sources, this unit is referred to as "pextp".
+
+properties:
+  compatible:
+    const: mediatek,mt7988-xfi-tphy
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    items:
+      - description: XFI PHY clock
+      - description: XFI register clock
+
+  clock-names:
+    items:
+      - const: xfipll
+      - const: topxtal
+
+  resets:
+    items:
+      - description: Reset controller corresponding to the phy instance.
+
+  mediatek,usxgmii-performance-errata:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      One instance of the T-PHY on MT7988 suffers from a performance
+      problem in 10GBase-R mode which needs a work-around in the driver.
+      This flag enables a work-around ajusting an analog phy setting and
+      is required for XFI Port0 of the MT7988 SoC to be in compliance with
+      the SFP specification.
+
+  "#phy-cells":
+    const: 0
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+  - resets
+  - "#phy-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/mediatek,mt7988-clk.h>
+    soc {
+      #address-cells = <2>;
+      #size-cells = <2>;
+
+      phy@11f20000 {
+        compatible = "mediatek,mt7988-xfi-tphy";
+        reg = <0 0x11f20000 0 0x10000>;
+        clocks = <&xfi_pll CLK_XFIPLL_PLL_EN>,
+                 <&topckgen CLK_TOP_XFI_PHY_0_XTAL_SEL>;
+        clock-names = "xfipll", "topxtal";
+        resets = <&watchdog 14>;
+        mediatek,usxgmii-performance-errata;
+        #phy-cells = <0>;
+      };
+    };
+
+...