OMAP2+: voltage: move VC into struct voltagedomain, misc. renames

Move the VC instance struct from omap_vdd_info into struct voltagedomain.
While moving, perform some misc. renames for readability.

No functional changes.

Summary of renames:
- rename omap_vc_instance to omap_vc_channel, since there is only
  one instance of the VC IP and this actually represents channels
  using TRM terminology.
- rename 'vc_common' field of VC channel which led to:
  s/vc->vc_common/vc->common/
- remove redundant '_data' suffix
- OMAP3: vc1 --> vc_mpu, vc2 --> vc_core
- omap_vc_bypass_scale_voltage() -> omap_vc_bypass_scale()

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

merge