rtc: max77686: Add an indirection level to access RTC registers

The max77686 driver is generic enough that can be used for other
Maxim RTC IP blocks but these might not have the same registers
layout so instead of accessing the registers directly, add a map
to translate offsets to the real registers addresses for each IP.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>