mfd/pcf50633-gpio.c: add MODULE_LICENSE

Add the missing MODULE_LICENSE("GPL").

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: Balaji Rao <balajirrao@openmoko.org>
Cc: Andy Green <andy@openmoko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>