mfd: add platform_data to mfd_cell

Adding platform_data to mfd_cell allows passing of platform data directly
to the platform_device created for each cell and thus reuse of existing
drivers.
On the other side it can be used as a hook to mfd_cell itself
removing the need in mfd_get_cell method.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>