MAINTAINERS: add more files to eth PHY

include/linux/linkmode.h and include/linux/mii.h
do not match anything in MAINTAINERS. Looks like
they should be under Ethernet PHY.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>