rtc: max77686: Add support for MAX20024/MAX77620 RTC IP

Maxim Semiconductor's PMIC MAX77686 has RTC IP which is
reused in the MAX77620/MAX20024 PMICs.

Add support for these devices in MAX77686 RTC driver. This
device does not have RTC alarm pending status outside of
RTC IP. The RTC IP is having separate I2C address for its
register access.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>