gpio: thunderx: Convert to immutable irq_chip

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

Whereas in most cases we put the gpiochip_[enable|disable]_irq()
calls in the .irq_mask() and .irq_unmask() callbacks, here
we need to put them in the .irq_enable() and .irq_disable()
callbacks, as this driver uses both .irq_mask() and
.irq_mask_ack().

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>