OMAP2/3 PRCM: don't export prm_*(), cm_*() functions

Device drivers and loadable modules should not be calling these
prm_* and cm_* functions, so stop exporting them.  Only core code
and device driver integration code (in arch/arm/*omap*) should
call these functions.

Signed-off-by: Paul Walmsley <paul@pwsan.com>