gpio: mockup: use irq_sim

Shrink the driver by removing the code dealing with dummy interrupts
and replacing it with calls to the irq_sim API.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>