MAINTAINERS: exfat: add myself as reviewer

I have been contributing to exfat for sometime and I would like to help
with code reviews as well.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>