regulator: Add mode mapping function to struct regulator_desc

The "regulator-initial-mode" and "regulator-mode" DT properties allows
to configure the regulator operating modes at startup or when a system
enters into a susend state.

But these properties use as valid values the operating modes supported
by each device while the core deals with the standard operating modes.
So a mapping function is needed to translate from the hardware specific
modes to the standard ones.

This mapping is a non-varying configuration for each regulator, so add
a function pointer to struct regulator_desc that will allow drivers to
define their callback to do the modes translation.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>