MIPS: ath25: Constify static irq_domain_ops

The only usage of these structs is to pass their address to either
irq_domain_add_tree() or irq_domain_create_linear(), both which takes
pointers to const struct irq_domain_ops. Make them const to allow the
compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>