selftests: ptp: Add Physical Hardware Clock test

Test the PTP Physical Hardware Clock functionality using the "phc_ctl" (a
part of "linuxptp").

The test contains three sub-tests:
* "settime" test
* "adjtime" test
* "adjfreq" test

"settime" test:
* set the PHC time to 0 seconds.
* wait for 120.5 seconds.
* check if PHC time equal to 120.XX seconds.

"adjtime" test:
* set the PHC time to 0 seconds.
* adjust the time by 10 seconds.
* check if PHC time equal to 10.XX seconds.

"adjfreq" test:
* adjust the PHC frequency to be 1% faster.
* set the PHC time to 0 seconds.
* wait for 100.5 seconds.
* check if PHC time equal to 101.XX seconds.

Usage:
$ ./phc.sh /dev/ptp<X>

It is possible to run a subset of the tests, for example:
* To run only the "settime" test:
$ TESTS="settime" ./phc.sh /dev/ptp<X>

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>