OMAP3+: VC: cleanup PMIC register address configuration

- support both voltage register address and command register address
  for each VC channel
- add fields for voltage register address (volra) and command register
  address (cmdra) to struct omap_vc_channel
- use VC/VP register access read/modify/write helper
- remove volra_shift field (use __ffs(mask) for shift value)
- I2C addresses 10-bit, change size to u16

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