mfd: max77620: Add core driver for MAX77620/MAX20024

MAX77620/MAX20024 are Power Management IC from the MAXIM.
It supports RTC, multiple GPIOs, multiple DCDC and LDOs,
watchdog, clock etc.

Add MFD drier to provides common support for accessing the
device; additional drivers is developed on respected subsystem
in order to use the functionality of the device.

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: Lee Jones <lee.jones@linaro.org>