irqchip: loongarch-cpu: add DT support

LoongArch is coming to support booting with FDT, so DT
support of this driver is desired.

Signed-off-by: Liu Peibao <liupeibao@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221114113824.1880-2-liupeibao@loongson.cn