irqchip/aspeed-vic: Constify irq_domain_ops

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

Cc: Joel Stanley <joel@jms.id.au>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>