drivers/rtc/rtc-pm8xxx.c: rework to support pm8941 rtc

drivers/rtc/rtc-pm8xxx.c: rework to support pm8941 rtc

Adds support for RTC device inside PM8941 PMIC.  The RTC in this PMIC
have two register spaces.  Thus the rtc-pm8xxx is slightly reworked to
reflect these differences.

The register set for different PMIC chips are selected on DT compatible
string base.

[[email protected]: coding-style fixes]
[[email protected]: simplify and fix locking in pm8xxx_rtc_set_time()]
Signed-off-by: Stanimir Varbanov <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Josh Cartwright <[email protected]>
Cc: Stanimir Varbanov <[email protected]>
Cc: Dan Carpenter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>