irqchip/loongson-eiointc: Add suspend/resume support

Add suspend/resume support for EIOINTC irqchip, which is needed for
upcoming suspend/hibernation.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221020073527.541845-3-chenhuacai@loongson.cn