irq/gpio: ixp4xx: Drop boardfile probe path

The boardfiles for IXP4xx have been deleted. Delete all the
quirks and code dealing with that boot path and rely solely on
device tree boot.

Fix some missing static keywords that the kernel test robot
was complaining about while we're at it.

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