ARM: rockchip: Add pmu-sram binding

Rockchip SRAM for pmu:

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The sram of pmu is used to store the function of resume from maskrom(the 1st

level loader). This is a common use of the "pmu-sram" because it keeps power

even in low power states in the system.

Required node properties:

- compatible : should be "rockchip,rk3288-pmu-sram"

- reg : physical base address and the size of the registers window

Example:

	sram@ff720000 {

		compatible = "rockchip,rk3288-pmu-sram", "mmio-sram";

		reg = <0xff720000 0x1000>;

	};