Each PPS source can be registered/deregistered into the system by using
special modules called "clients". They simply define the PPS sources'
attributes and implement the time signal registration mechanism.

This patch adds a special directory for such clients and adds a dummy
client that can be useful to test system integrity on real systems.

[akpm@linux-foundation.org: coding-style fixes]
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