mmc: DT: sdhci-brcmstb: Add device tree bindings

The example includes the properties required to enable UHS modes.
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/brcm,bcm7425-sdhci.txt

+* BROADCOM BRCMSTB/BMIPS SDHCI Controller
+
+This file documents differences between the core properties in mmc.txt
+and the properties used by the sdhci-brcmstb driver.
+
+NOTE: The driver disables all UHS speed modes by default and depends
+on Device Tree properties to enable them for SoC/Board combinations
+that support them.
+
+Required properties:
+- compatible: "brcm,bcm7425-sdhci"
+
+Refer to clocks/clock-bindings.txt for generic clock consumer properties.
+
+Example:
+
+	sdhci@f03e0100 {
+		compatible = "brcm,bcm7425-sdhci";
+		reg = <0xf03e0000 0x100>;
+		interrupts = <0x0 0x26 0x0>;
+		sdhci,auto-cmd12;
+		clocks = <&sw_sdio>;
+		sd-uhs-sdr50;
+		sd-uhs-ddr50;
+	};
+
+	sdhci@f03e0300 {
+		non-removable;
+		bus-width = <0x8>;
+		compatible = "brcm,bcm7425-sdhci";
+		reg = <0xf03e0200 0x100>;
+		interrupts = <0x0 0x27 0x0>;
+		sdhci,auto-cmd12;
+		clocks = <sw_sdio>;
+		mmc-hs200-1_8v;
+	};