net: ptp: create a mock-up PTP Hardware Clock driver

There are several cases where virtual net devices may benefit from
having a PTP clock, and these have to do with testing. I can see at
least netdevsim and veth as potential users of a common mock-up PTP
hardware clock driver.

The proposed idea is to create an object which emulates PTP clock
operations on top of the unadjustable CLOCK_MONOTONIC_RAW plus a
software-controlled time domain via a timecounter/cyclecounter and then
link that PHC to the netdevsim device.

The driver is fully functional for its intended purpose, and it
successfully passes the PTP selftests.

$ cd tools/testing/selftests/ptp/
$ ./phc.sh /dev/ptp2
TEST: settime [ OK ]
TEST: adjtime [ OK ]
TEST: adjfreq [ OK ]

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-7-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>