omap2: add OMAP2_PRM_BASE/OMAP2_CM_BASE #defines

Add symbolic constants for OMAP2_PRM_BASE and OMAP2_CM_BASE addresses.
On the 2420 and 2430 these are the same, but on later OMAP
architectures, they are different.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>