MAINTAINERS: add a sample ethtool section entry

I feel like we don't do a good enough keeping authors of driver
APIs around. The ethtool code base was very nicely compartmentalized
by Michal. Establish a precedent of creating MAINTAINERS entries
for "sections" of the ethtool API. Use Andrew and cable test as
a sample entry. The entry should ideally cover 3 elements:
a core file, test(s), and keywords. The last one is important
because we intend the entries to cover core code *and* reviews
of drivers implementing given API!

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250204215750.169249-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>