iommu/mediatek: Cleanup pericfg lookup flow

Since only the INFRA type IOMMU needs to modify register(s) in the
pericfg iospace, it's safe to drop the pericfg_comp_str NULL check;
also, directly assign the regmap handle to data->pericfg instead of
to the infracfg variable to improve code readability.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220616110830.26037-6-angelogioacchino.delregno@collabora.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>