This turns the FTGPIO010 irqchip immutable.

Tested on the D-Link DIR-685.

Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>