MAINTAINERS: Temporarily add myself to the IOMMU entry

Joerg is recovering from an injury, so temporarily add myself to the
IOMMU MAINTAINERS entry so that I'm more likely to get CC'd on patches
while I help to look after the tree for him.

Suggested-by: Joerg Roedel <joro@8bytes.org>
Link: https://lore.kernel.org/r/20201117100953.GR22888@8bytes.org
Signed-off-by: Will Deacon <will@kernel.org>