regulator: gpio: Document regulator-type property

Reported-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

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

@@ -10,6 +10,8 @@ Optional properties:
 - gpios			: GPIO group used to control voltage.
 - startup-delay-us	: Startup time in microseconds.
 - enable-active-high	: Polarity of GPIO is active high (default is low).
+- regulator-type	: Specifies what is being regulated, must be either
+			  "voltage" or "current", defaults to current.
 
 Any property defined as part of the core regulator binding defined in
 regulator.txt can also be used.