pps: Reserve a line discipline number for PPS

Add a new line discipline for "pulse per second" devices connected to
a serial port.

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