MAINTAINERS: create entry for ethtool MAC merge

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

Acked-by: Vladimir Oltean <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

MAINTAINERS

···
 F:	tools/testing/selftests/drivers/net/*/ethtool*
 K:	cable_test
 
+NETWORKING [ETHTOOL MAC MERGE]
+M:	Vladimir Oltean <[e-mail]>
+F:	net/ethtool/mm.c
+F:	tools/testing/selftests/drivers/net/hw/ethtool_mm.sh
+K:	ethtool_mm
+
 NETWORKING [GENERAL]
 M:	"David S. Miller" <[e-mail]>
 M:	Eric Dumazet <[e-mail]>