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

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/amlogic,meson-gx.txt

+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>;
+	};