TTY: HVC, add tty_port

And use kref from that. This means we need tty_port->ops->destruct to
properly free the structure. This is what destroy_hvc_struct used to
do so we leverage that.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>