OMAP3+: voltage: convert to PRM register access functions

Convert VC/VP register access to use PRM VC/VP accessor functions.  In
the process, move the read/write function pointers from vdd_info into
struct voltagedomain.

No functional changes.

Additional cleanup:
- remove prm_mod field from  VC/VP data structures, the PRM register
  access functions know which PRM module to use.

Signed-off-by: Kevin Hilman <khilman@ti.com>