mg_disk: seperate mg_disk.h again

eec94620 fold mg_disk.h into mg_disk.c,
but mg_disk platform driver needs private data for operation. This also
make mg_disk.c as machine independent. Seperate only needed structure and
defines to mg_disk.h

Signed-off-by: unsik Kim <donari75@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>