dt-bindings: power: supply: Add otg regulator binding

Add a binding for the regulator which controls the OTG chargepath switch.
The OTG switch gets its power from pm8941_5vs1, and that should be
expressed as a usb_otg_in-supply property in the DT node for the
charger driver.  The regulator name is "otg-vbus".

Signed-off-by: Tim Bird <tim.bird@sonymobile.com>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>