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.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: simplify and fix locking in pm8xxx_rtc_set_time()]
Signed-off-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Josh Cartwright <joshc@codeaurora.org>
Cc: Stanimir Varbanov <svarbanov@mm-sol.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>