mmc: sdhci-st: ST Microelectronics SDHCI binding documentation.

This patch adds the device tree binding documentation for the ST
SDHCI driver. It documents the differences between the core properties
described by mmc.txt and the properties used by the sdhci-st driver.
Signed-off-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NPeter Griffin <peter.griffin@linaro.org>
Acked-by: NLee Jones <lee.jones@linaro.org>
Acked-by: NMaxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sdhci-st.txt

+* STMicroelectronics sdhci-st MMC/SD controller
+
+This file documents the differences between the core properties in
+Documentation/devicetree/bindings/mmc/mmc.txt and the properties
+used by the sdhci-st driver.
+
+Required properties:
+- compatible :  Must be "st,sdhci"
+- clock-names : Should be "mmc"
+                See: Documentation/devicetree/bindings/resource-names.txt
+- clocks :      Phandle of the clock used by the sdhci controler
+                See: Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+Optional properties:
+- non-removable: non-removable slot
+                 See: Documentation/devicetree/bindings/mmc/mmc.txt
+- bus-width: Number of data lines
+                 See: Documentation/devicetree/bindings/mmc/mmc.txt
+
+Example:
+
+mmc0: sdhci@fe81e000 {
+	compatible	= "st,sdhci";
+	status		= "disabled";
+	reg		= <0xfe81e000 0x1000>;
+	interrupts	= <GIC_SPI 127 IRQ_TYPE_NONE>;
+	interrupt-names	= "mmcirq";
+	pinctrl-names	= "default";
+	pinctrl-0	= <&pinctrl_mmc0>;
+	clock-names	= "mmc";
+	clocks		= <&clk_s_a1_ls 1>;
+	bus-width       = <8>
+};