mfd: Rename platform_data field of mfd_cell to mfd_data

Rename the platform_data variable to imply a distinction between
common platform_data driver usage (typically accessed via
pdev->dev.platform_data) and the way MFD passes data down to
clients (using a wrapper named mfd_get_data).

All clients have already been changed to use the wrapper function,
so this can be a quick single-commit change that only touches things
in drivers/mfd.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>