power: regulator: Add a driver for the AXP PMIC drivevbus

power: regulator: Add a driver for the AXP PMIC drivevbus

AXP PMICs have a pin which can either report the USB VBUS state, or
driving a regulator that supplies USB VBUS. Add a regulator driver for
controlling this pin. The selection between input and output is done via
the x-powers,drive-vbus-en pin on the PMIC (parent) node.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>