gpio: gpio-xilinx: Reduce spinlock array to array

Changed spinlock array to single. It is preparation for irq support which
is shared between two channels that's why spinlock should be only one.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>