regulator: ab8500: Add set_mode/get_mode support

Signed-off-by: Ludovic Barré <ludovic.barre@stericsson.com>
Signed-off-by: Emeric Vigier <emeric.vigier@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Bengt JONSSON <bengt.g.jonsson@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>