OMAP3+: VP: move voltage scale function pointer into struct voltagedomain

Function pointer used for actual voltage scaling (e.g. VP force update
or VC bypass) is moved from omap_vdd_info into struct voltagedomain,
resulting in renames s/vdd->volt_scale/voltdm->scale/

No functional changes.

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