MIPS: Loongson64: Move files to the top-level directory

Current Loongson-3 code can share among all Loongson64 processors.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: chenhc@lemote.com
Cc: paul.burton@mips.com