regulator: ab8500-ext: New driver to control external regulators

The ABx500 is capable of controlling three external regulator supplies.
Most commonly on and off are supported, but if an external regulator
chipset or power supply supports high-power and low-power mode settings,
we can control those too.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>