Device-Tree bindings for mixer driver

Required properties:
- compatible: value should be one of the following:
	1) "samsung,exynos5-mixer" <DEPRECATED>
	2) "samsung,exynos4210-mixer"
	3) "samsung,exynos4212-mixer"
	4) "samsung,exynos5250-mixer"
	5) "samsung,exynos5420-mixer"

- reg: physical base address of the mixer and length of memory mapped
	region.
- interrupts: interrupt number to the cpu.
- clocks: list of clock IDs from SoC clock driver.
	a) mixer: Gate of Mixer IP bus clock.
	b) sclk_hdmi: HDMI Special clock, one of the two possible inputs of
               mixer mux.
	c) hdmi: Gate of HDMI IP bus clock, needed together with sclk_hdmi.

Example:

	mixer {
		compatible = "samsung,exynos5250-mixer";
		reg = <0x14450000 0x10000>;
		interrupts = <0 94 0>;
	};