dt-binding: mediatek-dwmac: add binding document for MediaTek MT2712 DWMAC

The commit adds the device tree binding documentation for the MediaTek DWMAC
found on MediaTek MT2712.
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mediatek-dwmac.txt

+MediaTek DWMAC glue layer controller
+
+This file documents platform glue layer for stmmac.
+Please see stmmac.txt for the other unchanged properties.
+
+The device node has following properties.
+
+Required properties:
+- compatible:  Should be "mediatek,mt2712-gmac" for MT2712 SoC
+- reg:  Address and length of the register set for the device
+- interrupts:  Should contain the MAC interrupts
+- interrupt-names: Should contain a list of interrupt names corresponding to
+	the interrupts in the interrupts property, if available.
+	Should be "macirq" for the main MAC IRQ
+- clocks: Must contain a phandle for each entry in clock-names.
+- clock-names: The name of the clock listed in the clocks property. These are
+	"axi", "apb", "mac_main", "ptp_ref" for MT2712 SoC
+- mac-address: See ethernet.txt in the same directory
+- phy-mode: See ethernet.txt in the same directory
+- mediatek,pericfg: A phandle to the syscon node that control ethernet
+	interface and timing delay.
+
+Optional properties:
+- mediatek,tx-delay-ps: TX clock delay macro value. Default is 0.
+	It should be defined for rgmii/rgmii-rxid/mii interface.
+- mediatek,rx-delay-ps: RX clock delay macro value. Default is 0.
+	It should be defined for rgmii/rgmii-txid/mii/rmii interface.
+Both delay properties need to be a multiple of 170 for fine-tune rgmii,
+range 0~31*170.
+Both delay properties need to be a multiple of 550 for coarse-tune rgmii,
+range 0~31*550.
+Both delay properties need to be a multiple of 550 for mii/rmii,
+range 0~31*550.
+
+- mediatek,fine-tune: boolean property, if present indicates that fine delay
+	is selected for rgmii interface.
+	If present, tx-delay-ps/rx-delay-ps is 170+/-50ps per stage.
+	Else tx-delay-ps/rx-delay-ps of coarse delay macro is 0.55+/-0.2ns per stage.
+	This property do not apply to non-rgmii PHYs.
+	Only coarse-tune delay is supported for mii/rmii PHYs.
+- mediatek,rmii-rxc: boolean property, if present indicates that the rmii
+	reference clock, which is from external PHYs, is connected to RXC pin
+	on MT2712 SoC.
+	Otherwise, is connected to TXC pin.
+- mediatek,txc-inverse: boolean property, if present indicates that
+	1. tx clock will be inversed in mii/rgmii case,
+	2. tx clock inside MAC will be inversed relative to reference clock
+	   which is from external PHYs in rmii case, and it rarely happen.
+- mediatek,rxc-inverse: boolean property, if present indicates that
+	1. rx clock will be inversed in mii/rgmii case.
+	2. reference clock will be inversed when arrived at MAC in rmii case.
+- assigned-clocks: mac_main and ptp_ref clocks
+- assigned-clock-parents: parent clocks of the assigned clocks
+
+Example:
+	eth: ethernet@1101c000 {
+		compatible = "mediatek,mt2712-gmac";
+		reg = <0 0x1101c000 0 0x1300>;
+		interrupts = <GIC_SPI 237 IRQ_TYPE_LEVEL_LOW>;
+		interrupt-names = "macirq";
+		phy-mode ="rgmii";
+		mac-address = [00 55 7b b5 7d f7];
+		clock-names = "axi",
+			      "apb",
+			      "mac_main",
+			      "ptp_ref",
+			      "ptp_top";
+		clocks = <&pericfg CLK_PERI_GMAC>,
+			 <&pericfg CLK_PERI_GMAC_PCLK>,
+			 <&topckgen CLK_TOP_ETHER_125M_SEL>,
+			 <&topckgen CLK_TOP_ETHER_50M_SEL>;
+		assigned-clocks = <&topckgen CLK_TOP_ETHER_125M_SEL>,
+				  <&topckgen CLK_TOP_ETHER_50M_SEL>;
+		assigned-clock-parents = <&topckgen CLK_TOP_ETHERPLL_125M>,
+					 <&topckgen CLK_TOP_APLL1_D3>;
+		mediatek,pericfg = <&pericfg>;
+		mediatek,tx-delay-ps = <1530>;
+		mediatek,rx-delay-ps = <1530>;
+		mediatek,fine-tune;
+		mediatek,rmii-rxc;
+		mediatek,txc-inverse;
+		mediatek,rxc-inverse;
+		snps,txpbl = <32>;
+		snps,rxpbl = <32>;
+		snps,reset-gpio = <&pio 87 GPIO_ACTIVE_LOW>;
+		snps,reset-active-low;
+	};