iommu/mediatek-v1: add missing larb count sanity check

iommu/mediatek-v1: add missing larb count sanity check

Add the missing larb count sanity check to avoid writing beyond a fixed
sized array in case of a malformed devicetree.

Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>