dt-bindings: sdhci-sprd: Add bindings for the sdhci-sprd controller

This patch adds the device-tree binding documentation for Spreadtrum
SDHCI driver.
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

+* Spreadtrum SDHCI controller (sdhci-sprd)
+
+The Secure Digital (SD) Host controller on Spreadtrum SoCs provides an interface
+for MMC, SD and SDIO types of cards.
+
+This file documents differences between the core properties in mmc.txt
+and the properties used by the sdhci-sprd driver.
+
+Required properties:
+- compatible: Should contain "sprd,sdhci-r11".
+- reg: physical base address of the controller and length.
+- interrupts: Interrupts used by the SDHCI controller.
+- clocks: Should contain phandle for the clock feeding the SDHCI controller
+- clock-names: Should contain the following:
+	"sdio" - SDIO source clock (required)
+	"enable" - gate clock which used for enabling/disabling the device (required)
+
+Optional properties:
+- assigned-clocks: the same with "sdio" clock
+- assigned-clock-parents: the default parent of "sdio" clock
+
+Examples:
+
+sdio0: sdio@20600000 {
+	compatible  = "sprd,sdhci-r11";
+	reg = <0 0x20600000 0 0x1000>;
+	interrupts = <GIC_SPI 60 IRQ_TYPE_LEVEL_HIGH>;
+
+	clock-names = "sdio", "enable";
+	clocks = <&ap_clk CLK_EMMC_2X>,
+		 <&apahb_gate CLK_EMMC_EB>;
+	assigned-clocks = <&ap_clk CLK_EMMC_2X>;
+	assigned-clock-parents = <&rpll CLK_RPLL_390M>;
+
+	bus-width = <8>;
+	non-removable;
+	no-sdio;
+	no-sd;
+	cap-mmc-hw-reset;
+	status = "okay";
+};