regulator: max77686: Document gpio properties

Document usage of maxim,ena-gpios properties which turn on external/GPIO
control over regulator.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/mfd/max77686.txt

···
 	-BUCKn	:	1-4.
   Use standard regulator bindings for it ('regulator-off-in-suspend').
 
+  LDO20, LDO21, LDO22, BUCK8 and BUCK9 can be configured to GPIO enable
+  control. To turn this feature on this property must be added to the regulator
+  sub-node:
+	- maxim,ena-gpios :	one GPIO specifier enable control (the gpio
+				flags are actually ignored and always
+				ACTIVE_HIGH is used)
 
 Example:
 
···
 				regulator-max-microvolt = <1300000>;
 				regulator-always-on;
 				regulator-boot-on;
+			};
+
+			buck9_reg {
+				regulator-compatible = "BUCK9";
+				regulator-name = "CAM_ISP_CORE_1.2V";
+				regulator-min-microvolt = <1000000>;
+				regulator-max-microvolt = <1200000>;
+				maxim,ena-gpios = <&gpm0 3 GPIO_ACTIVE_HIGH>;
 			};
 	}