regulator: twl4030: add support for external voltage get/set

This is needed for SMPS regulators, which use the OMAP voltage
processor for voltage get/set functions instead of the normal I2C
channel. For this purpose, regulator_init_data->driver_data contents
are expanded, it is now a struct which contains function pointers
for the set/get voltage operations, a data pointer for these, and
the previously used features bitmask.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com> [for the MFD part]
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>