nfp: add support for 'ethtool -t DEVNAME' command

Add support for ethtool selftest.

e.g.
 # ethtool -t DEVNAME

test result like:
The test result is PASS
The test extra info:
Link Test        0
NSP Test         0
Firmware Test    0
Register Test    0

Signed-off-by: Fei Qin <fei.qin@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>