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>

arch/mips/boot/dts/loongson/loongson64v_4core_virtio.dts

@@ -88,7 +88,7 @@ pci@1a000000 {
 			interrupt-map-mask = <0x1800 0x0 0x0  0x7>;
 		};
 
-		isa {
+		isa@18000000 {
 			compatible = "isa";
 			#address-cells = <2>;
 			#size-cells = <1>;