mfd: mfd_cell is now implicitly available to t7166xb drivers

No need to explicitly set the cell's platform_data/data_size.

Modify clients to use mfd_get_cell helper function instead of
accessing platform_data directly.

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