Like axp221, axp223, axp813 the axp803 is also supporting external
regulator to drive the  OTG VBus through N_VBUSEN PMIC pin.

Add support for it.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>