mips: dts: loongson: fix DTC unit name warnings

Fixes the following W=1 kernel build warning(s):
arch/mips/boot/dts/loongson/rs780e-pch.dtsi:24.7-41.5: Warning (unit_address_vs_reg): /bus@10000000/isa: node has a reg or ranges property, but no unit name
arch/mips/boot/dts/loongson/rs780e-pch.dtsi:24.7-41.5: 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/rs780e-pch.dtsi

@@ -21,7 +21,7 @@ pci@1a000000 {
 				 <0x02000000 0 0x40000000 0 0x40000000 0 0x40000000>;
 		};
 
-		isa {
+		isa@18000000 {
 			compatible = "isa";
 			#address-cells = <2>;
 			#size-cells = <1>;