mmc: Use device platform_data to retrieve tmio_mmc platform bits

With the addition of the platform device mfd_cell pointer, we can now
cleanly pass the sub device drivers platform data pointers through the
regular device platform_data one, and get rid of mfd_get_data()

Cc: Ian Molton <spyro@f2s.com>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>