Add new servo for NTP SHM reference clock.

This is a simple servo that provides the NTP SHM reference clock. It
doesn't make any clock adjustments and it always returns with the
unlocked state. It writes all samples to the SHM segment and another
process (e.g. chronyd or ntpd) is needed to read the samples and
actually synchronize the clock. The SHM segment number is set to the PTP
domain number to allow multiple SHM reference clocks running at the same
time.

This is mainly useful when other time sources are available on the
system (e.g. NTP, hardware reference clocks, or other PTP domains)
and a fallback to/from PTP is needed.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>