ARM: OMAP2: CM/PM: remove direct register accesses outside CM code

Users of the CM funtionality should not access the CM registers directly
by themselves. Thus, added new CM driver APIs for the OMAP2 specific
functionalities which support the existing direct register accesses, and
changed the platform code to use these. This is done in preparation
for moving the CM code into its own individual driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>