ntp: Adding pps support to ntpd

Added an option for the ntpd application to support pps inputs.

Signed-off-by: Bryan Brinsko <bryan.brinsko@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>