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

* 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";

};