rtc: pm8xxx: use regmap API for register accesses

Now that the parent mfd driver has been made to work again, and has been
reworked to create a regmap instance intended for its children to use,
rework the pm8xxx driver to use the regmap API for its register
accesses.

Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>