iommu/mediatek: Add error handle for mtk_iommu_probe

In the original code, we lack the error handle. This patch adds them.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-18-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>