dt-bindings: mfd: rn5t618: Document optional property system-power-controller

The RN5T618 family of PMICs can be used as system management
controllers, in which case they handle poweroff and restart. Document
this capability by referring to the corresponding generic DT binding.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/rn5t618.txt

@@ -14,6 +14,10 @@ Required properties:
 		"ricoh,rc5t619"
  - reg: the I2C slave address of the device
 
+Optional properties:
+ - system-power-controller:
+   See Documentation/devicetree/bindings/power/power-controller.txt
+
 Sub-nodes:
  - regulators: the node is required if the regulator functionality is
    needed. The valid regulator names are: DCDC1, DCDC2, DCDC3, DCDC4
@@ -28,6 +32,7 @@ Example:
 	pmic@32 {
 		compatible = "ricoh,rn5t618";
 		reg = <0x32>;
+		system-power-controller;
 
 		regulators {
 			DCDC1 {