ARM: OMAP2+: PRM: remove PRM weak functions

Remove the now-unused PRM weak functions from prm_common.c.  These
were formerly used to ensure that some OMAP2/3 PRM code would build on
OMAP4, but none of those functions ever would have worked on OMAP4 due
to an incompatible PRM register layout.  Now all that has been cleaned
up and these can be removed.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Russ Dill <Russ.Dill@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>