pm8xxx: core: Add pm8xxx functions to get PMIC version and revision

Add a pair of APIs in pm8xxx/core.h: pm8xxx_get_version which returns
an enum representing the type of the PMIC (8058, 8901, 8921, etc) and
pm8xxx_get_revision which returns the silicon revision of the PMIC
chip.

Update pm8921-core to implement this new API and remove core data
members previously used to pass revision information.

Change-Id: Ib2aaf5843e4aef9281745919908c530b85717510
Signed-off-by: David Collins <collinsd@codeaurora.org>