rdc321x-southbridge: Use mfd_data instead of driver_data

Use mfd_data for passing information from mfd drivers to soc
clients.  The mfd_cell's driver_data field is being phased out.

Clients that were using driver_data now access .mfd_data
via mfd_get_data().

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>