Remove OMAP_PRM_REGADDR and use processor specific defines instead.

Also fold in a patch from Kevin Hilman to add _OFFSET #defines
for the PRCM registers to be used with the prm_[read|write]_* macros.
These are used extensively in the forthcoming OMAP PM support.

Also remove now unused OMAP2_PRM_BASE.
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>