gpio: mxs: use devm_irq_alloc_descs()

This driver never frees the interrupt descriptors it allocates. Fix
it by using the resource managed version of irq_alloc_descs().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>