regulator: Use device platform_data to retrieve db8500 platform bits

With the addition of a platform device mfd_cell pointer, MFD drivers
can go back to passing platform data back to their sub drivers.
This allows for an mfd_cell->mfd_data removal and thus keep the
sub drivers MFD agnostic. This is mostly needed for non MFD aware
sub drivers.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>