mfd: mfd_cell is now implicitly available to ab3550 driver

No clients (in mainline kernel, I'm told that drivers exist in external
trees that are planned for mainline inclusion) make use of this, nor
do they make use of platform_data, so nothing really had to change here.

The .data_size field is unused, so its usage gets removed.

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