testptp: Add option to open PHC in readonly mode

PTP Hardware Clocks no longer require WRITE permission to perform
readonly operations, such as listing device capabilities or listening to
EXTTS events once they have been enabled by a process with WRITE
permissions.

Add '-r' option to testptp to open the PHC in readonly mode instead of
the default read-write mode. Skip enabling EXTTS if readonly mode is
requested.

Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Wojtek Wasko <wwasko@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>