gpio: xlp: Convert to immutable irq_chip

Convert the driver to immutable irq-chip with a bit of
intuition.

In this case the driver uses .mask_ack() and .unmask()
and since I have a vague idea about the semantics of
.mask_ack() I added .irq_enable() to the existing .irq_disable()
and called into the gpiolib core from those callbacks
instead of mask/unmask.

Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>