gpio: ixp4xx: Use irq_domain_translate_twocell()

Use standard functions. Eventually we will be able to drop all
checks and just assign irq_domain_translate_twocell() to this
callback.

Cc: Brian Masney <masneyb@onstation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>