mfd: rk808: dt: Add the description about dvs gpio for rk808

add the description about dvs1, dvs2, and add the example.
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/rk808.txt

@@ -24,6 +24,10 @@ Optional properties:
 - vcc10-supply: The input supply for LDO_REG6
 - vcc11-supply: The input supply for LDO_REG8
 - vcc12-supply: The input supply for SWITCH_REG2
+- dvs-gpios:  buck1/2 can be controlled by gpio dvs, this is GPIO specifiers
+  for 2 host gpio's used for dvs. The format of the gpio specifier depends in
+  the gpio controller. If DVS GPIOs aren't present, voltage changes will happen
+  very quickly with no slow ramp time.
 
 Regulators: All the regulators of RK808 to be instantiated shall be
 listed in a child node named 'regulators'. Each regulator is represented
@@ -55,7 +59,9 @@ Example:
 		interrupt-parent = <&gpio0>;
 		interrupts = <4 IRQ_TYPE_LEVEL_LOW>;
 		pinctrl-names = "default";
-		pinctrl-0 = <&pmic_int>;
+		pinctrl-0 = <&pmic_int &dvs_1 &dvs_2>;
+		dvs-gpios = <&gpio7 11 GPIO_ACTIVE_HIGH>,
+			    <&gpio7 15 GPIO_ACTIVE_HIGH>;
 		reg = <0x1b>;
 		rockchip,system-power-controller;
 		wakeup-source;