Adds support, by using the PPS line discipline, for the PPS sources
connected with the CD (Carrier Detect) pin of a serial port.

[akpm@linux-foundation.org: fix cast size warnings]
Signed-off-by: Rodolfo Giometti
Cc: David Woodhouse
Cc: Greg KH
Cc: Alan Cox
Cc: Alexander Gordeev
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds