Add PHC refclock driver

Add PHC refclock driver

Implement a driver which allows using PTP hardware clock (PHC) as a
reference clock. It uses the PTP_SYS_OFFSET ioctl or clock_gettime()
to measure the offset between the PTP clock and the system clock. Ten
readings are made for every driver poll and the fastest one is returned.

As PHCs are typically kept in TAI instead of UTC, it's necessary to set
the TAI/UTC offset manually by the offset option. This could be improved
by obtaining the offset automatically from the right/UTC timezone.