regulator: mt6397: Implement of_map_mode

Implementing of_map_mode is necessary to be able to specify operating
modes in the devicetree using 'regulator-allowed-modes', and to change
regulator modes.

Signed-off-by: Anand K Mistry <amistry@google.com>
Link: https://lore.kernel.org/r/20200702162231.v2.3.I7acfb591bfacf3b1b04a3d388385098bfcc9fecd@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>