dt-bindings: add MediaTek XS-PHY binding

Add a DT binding documentation of XS-PHY for MediaTek SoCs
with USB3.1 GEN2 controller
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-mtk-xsphy.txt

+MediaTek XS-PHY binding
+--------------------------
+
+The XS-PHY controller supports physical layer functionality for USB3.1
+GEN2 controller on MediaTek SoCs.
+
+Required properties (controller (parent) node):
+ - compatible	: should be "mediatek,<soc-model>-xsphy", "mediatek,xsphy",
+		  soc-model is the name of SoC, such as mt3611 etc;
+		  when using "mediatek,xsphy" compatible string, you need SoC specific
+		  ones in addition, one of:
+		  - "mediatek,mt3611-xsphy"
+
+ - #address-cells, #size-cells : should use the same values as the root node
+ - ranges: must be present
+
+Optional properties (controller (parent) node):
+ - reg		: offset and length of register shared by multiple U3 ports,
+		  exclude port's private register, if only U2 ports provided,
+		  shouldn't use the property.
+ - mediatek,src-ref-clk-mhz	: u32, frequency of reference clock for slew rate
+		  calibrate
+ - mediatek,src-coef	: u32, coefficient for slew rate calibrate, depends on
+		  SoC process
+
+Required nodes	: a sub-node is required for each port the controller
+		  provides. Address range information including the usual
+		  'reg' property is used inside these nodes to describe
+		  the controller's topology.
+
+Required properties (port (child) node):
+- reg		: address and length of the register set for the port.
+- clocks	: a list of phandle + clock-specifier pairs, one for each
+		  entry in clock-names
+- clock-names	: must contain
+		  "ref": 48M reference clock for HighSpeed analog phy; and 26M
+			reference clock for SuperSpeedPlus analog phy, sometimes is
+			24M, 25M or 27M, depended on platform.
+- #phy-cells	: should be 1
+		  cell after port phandle is phy type from:
+			- PHY_TYPE_USB2
+			- PHY_TYPE_USB3
+
+The following optional properties are only for debug or HQA test
+Optional properties (PHY_TYPE_USB2 port (child) node):
+- mediatek,eye-src	: u32, the value of slew rate calibrate
+- mediatek,eye-vrt	: u32, the selection of VRT reference voltage
+- mediatek,eye-term	: u32, the selection of HS_TX TERM reference voltage
+- mediatek,efuse-intr	: u32, the selection of Internal Resistor
+
+Optional properties (PHY_TYPE_USB3 port (child) node):
+- mediatek,efuse-intr	: u32, the selection of Internal Resistor
+- mediatek,efuse-tx-imp	: u32, the selection of TX Impedance
+- mediatek,efuse-rx-imp	: u32, the selection of RX Impedance
+
+Banks layout of xsphy
+-------------------------------------------------------------
+port        offset    bank
+u2 port0    0x0000    MISC
+            0x0100    FMREG
+            0x0300    U2PHY_COM
+u2 port1    0x1000    MISC
+            0x1100    FMREG
+            0x1300    U2PHY_COM
+u2 port2    0x2000    MISC
+            ...
+u31 common  0x3000    DIG_GLB
+            0x3100    PHYA_GLB
+u31 port0   0x3400    DIG_LN_TOP
+            0x3500    DIG_LN_TX0
+            0x3600    DIG_LN_RX0
+            0x3700    DIG_LN_DAIF
+            0x3800    PHYA_LN
+u31 port1   0x3a00    DIG_LN_TOP
+            0x3b00    DIG_LN_TX0
+            0x3c00    DIG_LN_RX0
+            0x3d00    DIG_LN_DAIF
+            0x3e00    PHYA_LN
+            ...
+
+DIG_GLB & PHYA_GLB are shared by U31 ports.
+
+Example:
+
+u3phy: usb-phy@11c40000 {
+	compatible = "mediatek,mt3611-xsphy", "mediatek,xsphy";
+	reg = <0 0x11c43000 0 0x0200>;
+	mediatek,src-ref-clk-mhz = <26>;
+	mediatek,src-coef = <17>;
+	#address-cells = <2>;
+	#size-cells = <2>;
+	ranges;
+
+	u2port0: usb-phy@11c40000 {
+		reg = <0 0x11c40000 0 0x0400>;
+		clocks = <&clk48m>;
+		clock-names = "ref";
+		mediatek,eye-src = <4>;
+		#phy-cells = <1>;
+	};
+
+	u3port0: usb-phy@11c43000 {
+		reg = <0 0x11c43400 0 0x0500>;
+		clocks = <&clk26m>;
+		clock-names = "ref";
+		mediatek,efuse-intr = <28>;
+		#phy-cells = <1>;
+	};
+};