Document PTP time scale usage and provide examples

Signed-off-by: Libor Pechacek <lpechacek@suse.cz>
phc2sys: Require either -O or -w on command line

The default zero offset can lead to misalignment between system clocks or wrong
time to be broadcast to the domain.  Therefore we require setting offset upon
invocation.

Signed-off-by: Libor Pechacek <lpechacek@suse.cz>