tsnep: Add free running cycle counter support

The TSN endpoint Ethernet MAC supports a free running counter
additionally to its clock. This free running counter can be read and
hardware timestamps are supported. As the name implies, this counter
cannot be set and its frequency cannot be adjusted.

Add free running cycle counter support based on this free running
counter to physical clock. This also requires hardware time stamps
based on that free running counter.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>