OMAP4: prcm: Add temporarily helper functions for rmw and read inside the PRM

Since OMAP4 is using an absolute address, the current PRM accessors
are not useable.
OMAP4 adaptation for these API are currently ongoing, so define temp
version until the proper ones are defined.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>