dt-bindings: arm: Hi6220: add doc for SRAM controller

Document "hisilicon,hi6220-sramctrl" for SRAM controller.
Signed-off-by: NLeo Yan <leo.yan@linaro.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>

Documentation/devicetree/bindings/arm/hisilicon/hisilicon.txt

@@ -127,6 +127,24 @@ Example:
 		#clock-cells = <1>;
 	};
 
+
+Hisilicon Hi6220 SRAM controller
+
+Required properties:
+- compatible : "hisilicon,hi6220-sramctrl", "syscon"
+- reg : Register address and size
+
+Hisilicon's SoCs use sram for multiple purpose; on Hi6220 there have several
+SRAM banks for power management, modem, security, etc. Further, use "syscon"
+managing the common sram which can be shared by multiple modules.
+
+Example:
+	/*for Hi6220*/
+	sram: sram@fff80000 {
+		compatible = "hisilicon,hi6220-sramctrl", "syscon";
+		reg = <0x0 0xfff80000 0x0 0x12000>;
+	};
+
 -----------------------------------------------------------------------
 Hisilicon HiP01 system controller