rtc: pm8xxx: use regmap API for register accesses

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 <[email protected]>
Reviewed-by: Stephen Boyd <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>