mfd: sta2x11-mfd: Use defines for platform devices' names

Since there are now many sta2x11-mfd platform devices, using defines
for their names looks like a better solution.

Signed-off-by: Davide Ciminaghi <ciminaghi@gnudd.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>