ARM: OMAP2+: powerdomain/PRM: move the low-level powerdomain functions into PRM

Move the low-level SoC-specific powerdomain control functions into
prm*.c.  For example, OMAP2xxx low-level powerdomain functions go into
prm2xxx.c.  Then remove the unnecessary powerdomain*xxx*.c files.

The objective is to centralize low-level PRM register accesses into
the prm*.[ch] files, and then to export an OMAP SoC-independent API to
higher-level OMAP power management code.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Vaibhav Hiremath <hvaibhav@ti.com>
Acked-by: Rajendra Nayak <rnayak@ti.com>
Reviewed-by: Russ Dill <Russ.Dill@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>