mmc: dt-bindings: add Mediatek MMC bindings

Document the device-tree binding of Mediatek MMC host
Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mtk-sd.txt

+* MTK MMC controller
+
+The MTK  MSDC can act as a MMC controller
+to support MMC, SD, and SDIO types of memory cards.
+
+This file documents differences between the core properties in mmc.txt
+and the properties used by the msdc driver.
+
+Required properties:
+- compatible: Should be "mediatek,mt8173-mmc","mediatek,mt8135-mmc"
+- interrupts: Should contain MSDC interrupt number
+- clocks: MSDC source clock, HCLK
+- clock-names: "source", "hclk"
+- pinctrl-names: should be "default", "state_uhs"
+- pinctrl-0: should contain default/high speed pin ctrl
+- pinctrl-1: should contain uhs mode pin ctrl
+- vmmc-supply: power to the Core
+- vqmmc-supply: power to the IO
+
+Examples:
+mmc0: mmc@11230000 {
+	compatible = "mediatek,mt8173-mmc", "mediatek,mt8135-mmc";
+	reg = <0 0x11230000 0 0x108>;
+	interrupts = <GIC_SPI 39 IRQ_TYPE_LEVEL_LOW>;
+	vmmc-supply = <&mt6397_vemc_3v3_reg>;
+	vqmmc-supply = <&mt6397_vio18_reg>;
+	clocks = <&pericfg CLK_PERI_MSDC30_0>, <&topckgen CLK_TOP_MSDC50_0_H_SEL>;
+	clock-names = "source", "hclk";
+	pinctrl-names = "default", "state_uhs";
+	pinctrl-0 = <&mmc0_pins_default>;
+	pinctrl-1 = <&mmc0_pins_uhs>;
+};