regulator: ab8500: Added get_optimum_mode on regulators with idle mode

With this change, Vtvout, Vintcore12, Vaux1, 2 and 3 regulators
support DRMS (Dynamic Regulator Mode Switching) which will
dynamically handle requests for max current consumption from
several consumers and select a suitable regulator mode.

Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>