fs: imgdafs: Add IMG DAFS filesystem for metag

Add the IMG Debug Adapter File System (DAFS) for metag, which uses
SWITCH operations to communicate with a file server on a host computer
via a JTAG debug adapter.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Joe Perches <joe@perches.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-fsdevel@vger.kernel.org