irqchip/loongson-eiointc: Add DT init support

Add EIOINTC irqchip DT support, which is needed for Loongson chips
based on DT and supporting EIOINTC, such as the Loongson-2K0500 SOC.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/764e02d924094580ac0f1d15535f4b98308705c6.1683279769.git.zhoubinbin@loongson.cn