OMAP2+: VC: support PMICs with separate voltage and command registers

The VC layer can support PMICs with separate voltage and command
registers by putting the different registers in the PRM_VC_SMPS_VOL_RA
and PRCM_VC_SMPS_CMD_RA registers respectively.

The PMIC data must supply at least a voltage register address
(volt_reg_addr).  The command register address (cmd_reg_addr) is
optional.  If the PMIC data does not supply a separate command
register address, the VC will use the voltage register address for both.

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