Add missing header.

Error log:
CC      drivers/input/serio/xilinx_ps2.o
drivers/input/serio/xilinx_ps2.c: In function 'xps2_of_probe':
drivers/input/serio/xilinx_ps2.c:249: error: implicit declaration of function 'of_address_to_resource'
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>