linuxptp: add phc_ctl program to help debug PHC devices

This is an updated version of a script I wrote a couple years ago for
debugging the PHC when writing a new driver. I figured that it might be
handy for the LinuxPTP project to include, as it can give some insight
into the PHC directly. I have updated it to make use of the shared code
here, in order to reduce duplication. Hopefully this is of some use to
everyone.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>