mfd: mfd_cell is now implicitly available to twl4030 drivers

The cell's platform_data is now accessed with a helper function;
change clients to use that, and remove the now-unused data_size.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>