pinctrl: sunxi: constify irq_domain_ops

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

Suggested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>