MIPS: Loongon64: DMA functions cleanup

Split the common dma.c which shared by Loongson-2E and Loongson-2F,
since the code in 'common' directory is assumed be shared by all 64bit
Loongson platforms (but Loongson-3 doesn't use it now). By the way,
Loongson-2E and Loongson-2F have already dropped 32bit kernel support,
so CONFIG_64BIT isn't needed.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20302/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: Huacai Chen <chenhuacai@gmail.com>