mips: dts: loongson: fix DTC unit name warnings

Fixes the following W=1 kernel build warning(s):
arch/mips/boot/dts/loongson/loongson64v_4core_virtio.dts:91.7-96.5: Warning (unit_address_vs_reg): /bus@10000000/isa: node has a reg or ranges property, but no unit name
arch/mips/boot/dts/loongson/loongson64v_4core_virtio.dts:91.7-96.5: Warning (simple_bus_reg): /bus@10000000/isa: simple-bus unit address format error, expected "18000000"

Signed-off-by: zhaoxiao <zhaoxiao@uniontech.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>