regulator: ltc3589: Add DT binding documentation

This patch adds the device tree binding documentation for Linear
Technology LTC3589, LTC3589-1, and LTC3589-2 8-port regulators.
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: NMark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/regulator/ltc3589.txt

+Linear Technology LTC3589, LTC3589-1, and LTC3589-2 8-output regulators
+
+Required properties:
+- compatible: "lltc,ltc3589", "lltc,ltc3589-1" or "lltc,ltc3589-2"
+- reg: I2C slave address
+
+Required child node:
+- regulators: Contains eight regulator child nodes sw1, sw2, sw3, bb-out,
+  ldo1, ldo2, ldo3, and ldo4, specifying the initialization data as
+  documented in Documentation/devicetree/bindings/regulator/regulator.txt.
+
+Each regulator is defined using the standard binding for regulators. The
+nodes for sw1, sw2, sw3, bb-out, ldo1, and ldo2 additionally need to specify
+the resistor values of their external feedback voltage dividers:
+
+Required properties (not on ldo3, ldo4):
+- lltc,fb-voltage-divider: An array of two integers containing the resistor
+  values R1 and R2 of the feedback voltage divider in ohms.
+
+Regulators sw1, sw2, sw3, and ldo2 can regulate the feedback reference from
+0.3625 V to 0.75 V in 12.5 mV steps. The output voltage thus ranges between
+0.3625 * (1 + R1/R2) V and 0.75 * (1 + R1/R2) V. Regulators bb-out and ldo1
+have a fixed 0.8 V reference and thus output 0.8 * (1 + R1/R2) V. The ldo3
+regulator is fixed to 1.8 V on LTC3589 and to 2.8 V on LTC3589-1,2. The ldo4
+regulator can output between 1.8 V and 3.3 V on LTC3589 and between 1.2 V
+and 3.2 V on LTC3589-1,2 in four steps. The ldo1 standby regulator can not
+be disabled and thus should have the regulator-always-on property set.
+
+Example:
+
+	ltc3589: pmic@34 {
+		compatible = "lltc,ltc3589-1";
+		reg = <0x34>;
+
+		regulators {
+			sw1_reg: sw1 {
+				regulator-min-microvolt = <591930>;
+				regulator-max-microvolt = <1224671>;
+				lltc,fb-voltage-divider = <100000 158000>;
+				regulator-ramp-delay = <7000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			sw2_reg: sw2 {
+				regulator-min-microvolt = <704123>;
+				regulator-max-microvolt = <1456803>;
+				lltc,fb-voltage-divider = <180000 191000>;
+				regulator-ramp-delay = <7000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			sw3_reg: sw3 {
+				regulator-min-microvolt = <1341250>;
+				regulator-max-microvolt = <2775000>;
+				lltc,fb-voltage-divider = <270000 100000>;
+				regulator-ramp-delay = <7000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			bb_out_reg: bb-out {
+				regulator-min-microvolt = <3387341>;
+				regulator-max-microvolt = <3387341>;
+				lltc,fb-voltage-divider = <511000 158000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			ldo1_reg: ldo1 {
+				regulator-min-microvolt = <1306329>;
+				regulator-max-microvolt = <1306329>;
+				lltc,fb-voltage-divider = <100000 158000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			ldo2_reg: ldo2 {
+				regulator-min-microvolt = <704123>;
+				regulator-max-microvolt = <1456806>;
+				lltc,fb-voltage-divider = <180000 191000>;
+				regulator-ramp-delay = <7000>;
+				regulator-boot-on;
+				regulator-always-on;
+			};
+
+			ldo3_reg: ldo3 {
+				regulator-min-microvolt = <2800000>;
+				regulator-max-microvolt = <2800000>;
+				regulator-boot-on;
+			};
+
+			ldo4_reg: ldo4 {
+				regulator-min-microvolt = <1200000>;
+				regulator-max-microvolt = <3200000>;
+			};
+		};
+	};