MAINTAINERS: Add entry for MediaTek SMI

I am the author of MediaTek SMI driver, and will to maintain
and develop it further.
Add myself to cover these items.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210914113703.31466-14-yong.wu@mediatek.com


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>