riscv: ae350: dts: Remove the unnecessary #address-cells in plic nodes

PLIC nodes don't have child nodes, so #address-cells is not needed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>