regulator: da9121: add mode support

Adds get/set for mode, and mapping from REGULATOR_MODE_* to select
PFM/PWM/Auto operation.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/7844c8f6facb6f7c0649381629cc75ccad14723d.1606755367.git.Adam.Ward.opensource@diasemi.com


Signed-off-by: Mark Brown <broonie@kernel.org>