regulator: hi655x: Document for hi655x regulator

This patch adds the device tree binding documentation for
hi655x PMIC regulator.
Signed-off-by: NChen Feng <puck.chen@hisilicon.com>
Signed-off-by: NFei Wang <w.f@huawei.com>
Signed-off-by: NXinwei Kong <kong.kongxinwei@hisilicon.com>
Reviewed-by: NHaojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/hisilicon,hi655x-regulator.txt

+Hisilicon Hi655x Voltage regulators
+
+Note:
+The Hi655x regulator control is managed by Hi655x PMIC.
+So the node of this regulator must be child node of Hi655x
+PMIC node.
+
+The driver uses the regulator core framework, so please also
+take the bindings of regulator.txt for reference.
+
+The valid names for regulators are:
+
+LDO2_2V8 LDO7_SDIO LDO10_2V85 LDO13_1V8 LDO14_2V8
+LDO15_1V8 LDO17_2V5 LDO19_3V0 LDO21_1V8 LDO22_1V2
+
+Example:
+        pmic: pmic@f8000000 {
+                compatible = "hisilicon,hi655x-pmic";
+		...
+		regulators {
+			ldo2: LDO2@a21 {
+				regulator-name = "LDO2_2V8";
+				regulator-min-microvolt = <2500000>;
+				regulator-max-microvolt = <3200000>;
+				regulator-enable-ramp-delay = <120>;
+			};
+			...
+		}
+	}