iommu/mediatek: Improve comment for the current region/bank

No functional change. Just add more comment about the current region/bank
in the code.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230411093144.2690-4-yong.wu@mediatek.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>