OMAP2+: voltage: move PRCM mod offets into VC/VP structures

Eliminate need for global variables for the various PRM module offsets by
making them part of the VP/VC common structures

Eventually, these will likely be moved again, or more likely removed
when VP/VC code is isolated, but for now just getting rid of them as
global variabes so that the voltage domain initialization can be
cleaned up.

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