ARM: rockchip: Add pmu-sram binding

The pmu-sram is used to store resume code, suspend/resume need get the
address of it. Therefore add a binding and documentation for it.
Signed-off-by: Tony Xie <xxx@rock-chips.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip/pmu-sram.txt

+Rockchip SRAM for pmu:
+------------------------------
+
+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>;
+	};