dt-bindings: regulator: Add active discharge support for stpmic1

Add support for active discharge for USB power switches.
Signed-off-by: Npascal paillet <p.paillet@st.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/st,stpmic1-regulator.txt

@@ -30,6 +30,7 @@ Optional properties:
 - interrupts: index of current limit detection interrupt
 - <regulator>-supply: phandle to the parent supply/regulator node
 	each regulator supply can be described except vref_ddr.
+- regulator-active-discharge: can be used on pwr_sw1 and pwr_sw2.
 
 Example:
 regulators {