irqchip: Add driver for Loongson-1 interrupt controller

This controller appeared on Loongson-1 family MCUs
including Loongson-1B and Loongson-1C.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>