MAINTAINERS: add Sean/Nelson as MediaTek ethernet maintainers

Sean and Nelson work for MediaTek on maintaining the MediaTek ethernet
driver for the existing SoCs and adding support for the following SoCs.
In the past, Sean has been active at making most of the qualifications
, stress test and submitting a lot of patches for the driver while
Nelson was looking into the aspects more on hardware additions and details
such as introducing PDMA with Hardware LRO to the driver. Also update
John's up-to-date mail address in the patch.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Nelson Chang <nelson.chang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>