tty: Introduce a tty_port common structure

Every tty driver has its own concept of a port structure and because
they all differ we cannot extract commonality.  Begin fixing this by
creating a structure drivers can elect to use so that over time we can
push fields into this and create commonality and then introduce common
methods.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>