etrax: enable arbitary speed setting on tty ports

Add the needed constants and bits. The actual code is already in the tty
layer and turned on by the definitions

Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Mikael Starvik <starvik@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>