tty/metag_da: Add metag DA TTY driver

Add a TTY driver for communicating over a Meta DA (Debug Adapter)
channel using the bios channel SWITCH operation.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jiri Slaby <jslaby@suse.cz>
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>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>