OMAP2+: PRM: add register access functions for VC/VP

On OMAP3+, the voltage controller (VC) and voltage processor (VP) are
inside the PRM.  Add some PRM helper functions for register access to
these module registers.

Thanks to Nishanth Menon for finding/fixing a sparse problem.

Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>