Input: xilinx_ps2 - make Open Firmware device ids constant

The match_table field of the struct of_device_id is constant
in <linux/of_platform.h> so it makes sense to mark xps2_of_match
also constant.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>