regulator: axp20x: Add support for the (external) drivebus regulator

The axp20x pmics have 2 power inputs, one called ACIN which is intended
for to be supplied via a powerbarrel on the board and one called VBUS
which is intended to be supplied via an otg connector.

In the VBUS case the pmic needs to know if the board is supplying power
to the otg connector, because then it should not take any power from
its VBUS pin. The axp209 pmic has a N_VBUSEN input pin via which the
board can signal to the pmic whether the board is supplying power to the
otg connector or not.

On the axp221/axp223 this pin can alternatively be used as an output
which controls an external regulator which (optionally) supplies
power to the otg connector from the board. When the pin is used as
output it is called DRIVEVBUS in the datasheet.

This commit adds support for the DRIVEVBUS pin as an extra pmic
controlled regulator. Since this is optional a new x-powers,drivebus dt
property is added. When this is present the misc-control register is
written to change the N_VBUSEN input pin to DRIVEVBUS output pin mode and
the extra drivebus regulator is registered with the regulator subsystem.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>