regulator: tps65090: add external control support for DCDC

The TPS65090's DCDC output can also be enable/disable through the
external digital input signal. Add support for enable/disable
either through register access via I2C or through external
control inputs. The external control inputs can be driven through
GPIOs also and hence adding support for passing the GPIO number.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>