irqchip: ingenic: Alloc generic chips from IRQ domain

By creating the generic chips from the IRQ domain, we don't rely on the
JZ4740_IRQ_BASE macro. It also makes the code a bit cleaner.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1570015525-27018-5-git-send-email-zhouyanjie@zoho.com