gpio: mockup: use devm_irq_alloc_descs()

Use the resource managed variant of irq_alloc_descs(). This allows us
to remove gpio_mockup_remove().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>