rtc: pm8xxx: add support for devicetree

Add support for describing the PM8921/PM8058 RTC in device tree.

Additionally:
   - drop support for describing the RTC using platform data,
     as there are no current in tree users who do so.
   - make allow_set_time a device-specific flag, instead of mucking
     with the rtc_ops

Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Lee Jones <lee.jones@linaro.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>