net-next: mediatek: add an entry to MAINTAINERS

Add myself and Felix as the Maintainers for the MediaTek ethernet driver.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>