selftests/ptp: Add -x option for testing PTP_SYS_OFFSET_EXTENDED

The -x option (where 'x' stands for eXtended) takes an argument which
represents the number of samples to request from the PTP device.
The help message will display the maximum number of samples allowed.
Providing an invalid argument will also display the maximum number of
samples allowed.

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