mmc: sdhci-iproc: add device tree bindings

Add device tree binding documentation for IPROC SDHCI driver.
Acked-by: NRay Jui <rjui@broadcom.com>
Signed-off-by: NCorneliu Doban <cdoban@broadcom.com>
Signed-off-by: NScott Branden <sbranden@broadcom.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

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

+Broadcom IPROC SDHCI controller
+
+This file documents differences between the core properties described
+by mmc.txt and the properties that represent the IPROC SDHCI controller.
+
+Required properties:
+- compatible : Should be "brcm,sdhci-iproc-cygnus".
+- clocks : The clock feeding the SDHCI controller.
+
+Optional properties:
+  - sdhci,auto-cmd12: specifies that controller should use auto CMD12.
+
+Example:
+
+sdhci0: sdhci@0x18041000 {
+	compatible = "brcm,sdhci-iproc-cygnus";
+	reg = <0x18041000 0x100>;
+	interrupts = <GIC_SPI 108 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&lcpll0_clks BCM_CYGNUS_LCPLL0_SDIO_CLK>;
+	bus-width = <4>;
+	sdhci,auto-cmd12;
+	no-1-8-v;
+};