dt-bindings: Document the property poweroff-source for act8865 regulator

Signed-off-by: NRomain Perier <romain.perier@gmail.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/act8865-regulator.txt

@@ -5,6 +5,10 @@ Required properties:
 - compatible: "active-semi,act8846" or "active-semi,act8865"
 - reg: I2C slave address
 
+Optional properties:
+- poweroff-source: Telling whether or not this pmic is controlling
+  the system power. See Documentation/devicetree/bindings/power/poweroff.txt .
+
 Any standard regulator properties can be used to configure the single regulator.
 
 The valid names for regulators are: