MIPS: Loongson: Switch from flatmem to sparsemem

With flatmem hibernation for Loongson will fail, and there are also some
other problems such as broken files when using NFS or CIFS / Samba.

The config help of sparsemem says:

"This option provides some potential performance benefits, along with
decreased code complexity."

So to avoid the potential problems of FLATMEM, we disable FLATMEM directly
and use SPARSEMEM instead.

Related email thread:

http://groups.google.com/group/loongson-dev/browse_thread/thread/b6b65890ec2b0f24/feb43e5aa7f55d9b?show_docid=feb43e5aa7f55d9b

Reported-by: Tatu Kilappa <tatu.kilappa@gmail.com>
Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/737/
Cc: linux-mips@linux-mips.org
Cc: zhangfx@lemote.com
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>