iommu/mediatek: Check return value after calling platform_get_resource()

platform_get_resource() may return NULL pointer, we need check its
return value to avoid null-ptr-deref in resource_size().

Fixes: 42d57fc5 ("iommu/mediatek: Initialise/Remove for multi bank dev")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20221029103550.3774365-1-yangyingliang@huawei.com


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