dt: bindings: Add bindings for SDHCI Synopsys DWC MSHC

* Synopsys DesignWare Cores Mobile Storage Host Controller

Required properties:

- compatible: should be one of the following:

    "snps,dwcmshc-sdhci"

- reg: offset and length of the register set for the device.

- interrupts: a single interrupt specifier.

- clocks: Array of clocks required for SDHCI; requires at least one for

    core clock.

- clock-names: Array of names corresponding to clocks property; shall be

    "core" for core clock and "bus" for optional bus clock.

Example:

	sdhci2: sdhci@aa0000 {

		compatible = "snps,dwcmshc-sdhci";

		reg = <0xaa0000 0x1000>;

		interrupts = <GIC_SPI 21 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&emmcclk>;

		bus-width = <8>;

	}