ARM: OMAP2+: PRM: split PRM functions into OMAP2, OMAP3-specific files

Move OMAP3xxx-specific PRM functions & macros into prm3xxx.[ch] and
OMAP2xxx-specific macros into prm2xxx.h.  (prm2xxx.c will be created
by a subsequent patch when it's needed.)  Move basic PRM register
access functions into static inline functions in prm2xxx_3xxx.h, leaving
only OMAP2/3 hardreset functions in prm2xxx_3xxx.c.

Also clarify the initcall function naming to reinforce that this code
is specifically for the PRM IP block.

This is in preparation for the upcoming powerdomain series and the
upcoming move of this code to drivers/.

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