struct irq_domain_ops is not modified, so it can be made const.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: uclinux-h8-devel@lists.sourceforge.jp
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>