regulator: max77802: Add .{get,set}_mode callbacks

Some max77802 LDOs (1, 3, 20 and 21) support to be configured in Low
Power Mode during system normal operation. Add function handlers for
the .get_mode and .set_mode operations to set the mode on these LDOs.

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