MIPS: Loongson64: Fix machine naming

From previous commits, the machine names with "loongson3-" prefix have
renamed to "loongson64c-" prefix in documents, but the .dts files have
not been updated as well. So fix it.
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/loongson/loongson64c_4core_rs780e.dts

@@ -6,7 +6,7 @@
 #include "rs780e-pch.dtsi"
 
 / {
-	compatible = "loongson,loongson3-4core-rs780e";
+	compatible = "loongson,loongson64c-4core-rs780e";
 };
 
 &package0 {

arch/mips/boot/dts/loongson/loongson64c_8core_rs780e.dts

@@ -6,7 +6,7 @@
 #include "rs780e-pch.dtsi"
 
 / {
-	compatible = "loongson,loongson3-8core-rs780e";
+	compatible = "loongson,loongson64c-8core-rs780e";
 };
 
 &package0 {