platform: andes/ae350: Use fdt irqchip driver

Andes PLIC is compatible with plic driver. The PLIC base address and
number of source can be obtained by parsing the device tree.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>