regulator: ab8500-ext: Add HW request support

Support for HW request is added in the external regulator
driver. A flag in the board configuration can be set to
let HW control the regulator when there is no SW request.
This means that the regulator will be put in high power
mode when there is a SW request and in HW-request mode
otherwise.

Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mattias NILSSON <mattias.i.nilsson@stericsson.com>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>