gpio: xilinx: Make of_device_id array const

Make of_device_id array const, because all OF functions
handle it as const.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>