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>