irqchip/ativic32: Constify irq_domain_ops

This is passed to irq_domain_add_linear(), which accepts a pointer
to a const structure.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200714173857.477422-1-masahiroy@kernel.org