MAINTAINERS: mt76: add git tree

Felix has a git tree for mt76 patches, document that.

Signed-off-by: Kalle Valo <kvalo@kernel.org>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230614144704.505553-1-kvalo@kernel.org