mfd: Add MODULE_DEVICE_TABLE to pcf50633-core

The device table is required to load modules based on modaliases.
After adding the MODULE_DEVICE_TABLE, below entries will be added to
modules.alias:
alias i2c:pcf50633 pcf50633

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>