mmc: dt-bindings: add ZTE ZX296718 MMC bindings

Document the device-tree binding of ZTE MMC host on
ZX296718 SoC.
Signed-off-by: NJun Nie <jun.nie@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/zx-dw-mshc.txt

+* ZTE specific extensions to the Synopsys Designware Mobile Storage
+  Host Controller
+
+The Synopsys designware mobile storage host controller is used to interface
+a SoC with storage medium such as eMMC or SD/MMC cards. This file documents
+differences between the core Synopsys dw mshc controller properties described
+by synopsys-dw-mshc.txt and the properties used by the ZTE specific
+extensions to the Synopsys Designware Mobile Storage Host Controller.
+
+Required Properties:
+
+* compatible: should be
+	- "zte,zx296718-dw-mshc": for ZX SoCs
+
+Example:
+
+	mmc1: mmc@1110000 {
+		compatible = "zte,zx296718-dw-mshc";
+		reg = <0x01110000 0x1000>;
+		interrupts = <GIC_SPI 15 IRQ_TYPE_LEVEL_HIGH>;
+		fifo-depth = <32>;
+		data-addr = <0x200>;
+		fifo-watermark-aligned;
+		bus-width = <4>;
+		clock-frequency = <50000000>;
+		clocks = <&topcrm SD0_AHB>, <&topcrm SD0_WCLK>;
+		clock-names = "biu", "ciu";
+		num-slots = <1>;
+		max-frequency = <50000000>;
+		cap-sdio-irq;
+		cap-sd-highspeed;
+		status = "disabled";
+	};