MAINTAINERS: Add entry for MediaTek IOMMU

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

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-34-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>