gpiolib: use irq_domain_alloc_irqs()

Use the irq_domain_alloc_irqs() wrapper instead of the full
__irq_domain_alloc_irqs() interface, which was only intended for some
legacy (x86) use cases.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>