mfd: Fetch cell pointer from platform_device->mfd_cell

In order for MFD drivers to fetch their cell pointer but also their
platform data one, an mfd cell pointer is added to the platform_device
structure.
That allows all MFD sub devices drivers to be MFD agnostic, unless
they really need to access their MFD cell data. Most of them don't,
especially the ones for IPs used by both MFD and non MFD SoCs.

Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Greg KH <gregkh@suse.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>