cns3xxx: Adopt irq_domain support for cns3xxx gpio driver

Have gpio driver adopt irqdomain support so that there are
non-overlapping allocations of irq numbers mapped to gpio's.

Signed-off-by: Pushpal Sidhu <psidhu@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42844 3c298f89-4303-0410-b956-a3cf2f4a3e73