irqchip/nvic: Use irq_domain_translate_onecell instead of custom func

Make use of newly introduced irq_domain_translate_onecell() instead of
custom made function.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1575976274-13487-3-git-send-email-yash.shah@sifive.com