selftest/ptp: update ptp selftest to exercise the gettimex options

selftest/ptp: update ptp selftest to exercise the gettimex options

With the inclusion of commit c259acab839e ("ptp/ioctl: support
MONOTONIC{,_RAW} timestamps for PTP_SYS_OFFSET_EXTENDED") clock_gettime()
now allows retrieval of pre/post timestamps for CLOCK_MONOTONIC and
CLOCK_MONOTONIC_RAW timebases along with the previously supported
CLOCK_REALTIME.

This patch adds a command line option 'y' to the testptp program to
choose one of the allowed timebases [realtime aka system, monotonic,
and monotonic-raw).

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://patch.msgid.link/20241003101506.769418-1-maheshb@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>