regulator: max77503: Add ADI MAX77503 support

Add ADI MAX77503 buck converter driver support.

Signed-off-by: Gokhan Celik <gokhan.celik@analog.com>
Link: https://lore.kernel.org/r/10bb3894fea31a098d768e346c8721e730d7cb21.1698000185.git.gokhan.celik@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>