Documentation: DT: MMC: meson-gx: new bindings doc

Amlogic SD / eMMC controller for S905/GXBB family SoCs

The MMC 5.1 compliant host controller on Amlogic provides the

interface for SD, eMMC and SDIO devices.

This file documents the properties in addition to those available in

the MMC core bindings, documented by mmc.txt.

Required properties:

- compatible : contains one of:

  - "amlogic,meson-gx-mmc"

  - "amlogic,meson-gxbb-mmc"

  - "amlogic,meson-gxl-mmc"

  - "amlogic,meson-gxm-mmc"

- clocks     : A list of phandle + clock-specifier pairs for the clocks listed in clock-names.

- clock-names: Should contain the following:

	"core" - Main peripheral bus clock

	"clkin0" - Parent clock of internal mux

	"clkin1" - Other parent clock of internal mux

  The driver has an interal mux clock which switches between clkin0 and clkin1 depending on the

  clock rate requested by the MMC core.

Example:

	sd_emmc_a: mmc@70000 {

		compatible = "amlogic,meson-gxbb-mmc";

		reg = <0x0 0x70000 0x0 0x2000>;

		interrupts = < GIC_SPI 216 IRQ_TYPE_EDGE_RISING>;

		clocks = <&clkc CLKID_SD_EMMC_A>, <&xtal>, <&clkc CLKID_FCLK_DIV2>;

		clock-names = "core", "clkin0", "clkin1";

		pinctrl-0 = <&emmc_pins>;

	};