mfd: Allow multiple MFD cells with the same name

Provide basic support for MFDs having multiple cells of a given
type with different IDs by adding an id to the mfd_cell structure
and then adding that to the id passed in to mfd_add_devices().

As it stands this approach requires that MFDs using this feature
deal with ensuring that there aren't any ID collisions resulting
from multiple MFDs of the same type being instantiated. This needs
to happen with the existing code too, but with this approach there
is a knock on effect on the IDs for non-duplicated devices.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>