regulator: max77620: add regulator driver for max77620/max20024

MAXIM Semiconductor's PMIC, MAX77620 and MAX20024 have the
multiple DCDC and LDOs. This supplies the power to different
components of the system.
Also these rails has configuration for ramp time, flexible
power sequence, slew rate etc.

Add regulator driver to access these rails via regulator APIs.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mallikarjun Kasoju <mkasoju@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>