selftests/ptp: Add -X option for testing PTP_SYS_OFFSET_PRECISE

The -X option was chosen because X looks like a cross, and the underlying
callback is 'get cross timestamp'.

Signed-off-by: Alex Maftei <alex.maftei@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>