net: dsa: mediatek: update MAINTAINERS entry with MediaTek switch driver

I work for MediaTek and maintain SoC targeting to home gateway and
also will keep extending and testing the function from MediaTek
switch.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>