regulator: Add DT bindings for tps65218 PMIC regulators.

Add DT bindings for tps65218 PMIC regulators.
Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/regulator/tps65218.txt

+TPS65218 family of regulators
+
+Required properties:
+For tps65218 regulators/LDOs
+- compatible:
+  - "ti,tps65218-dcdc1" for DCDC1
+  - "ti,tps65218-dcdc2" for DCDC2
+  - "ti,tps65218-dcdc3" for DCDC3
+  - "ti,tps65218-dcdc4" for DCDC4
+  - "ti,tps65218-dcdc5" for DCDC5
+  - "ti,tps65218-dcdc6" for DCDC6
+  - "ti,tps65218-ldo1" for LDO1
+
+Optional properties:
+- Any optional property defined in bindings/regulator/regulator.txt
+
+Example:
+
+	xyz: regulator@0 {
+		compatible = "ti,tps65218-dcdc1";
+		regulator-min-microvolt  = <1000000>;
+		regulator-max-microvolt  = <3000000>;
+	};