rtc: s5m: Add separate field for storing auto-cleared mask in register config

Some devices from S2M/S5M family use different register update masks for
different operations (alarm and register update). Now the driver uses
common register configuration and a lot of exceptions per device in code.

Before eliminating the exceptions and using specific register
configuration for given device, make the auto-cleared mask a separate
field. This is merely a refactoring.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>