gpio: ixp4xx: Make irqchip immutable

This turns the IXP4xx GPIO irqchip into an immutable
irqchip, a bit different from the standard template due
to being hierarchical.

Tested on the IXP4xx which uses drivers/ata/pata_ixp4xx_cf.c
for a rootfs on compact flash with IRQs from this GPIO
block to the CF ATA controller.

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>