MAINTAINERS: create entry for ethtool MAC merge

Vladimir implemented the MAC merge support and reviews all
the new driver implementations.

Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250215225200.2652212-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>