kernel: iommu: Don't reserve 0-length IOVA region

A flaw was found in the Linux kernel. When the bootloader/firmware does not set up the framebuffers, the address and size are 0 in the "iommu-addresses" property. If the IOVA region is reserved with a 0 length, it corrupts the IOVA rbtree with an entry that has pfnhi pfnlo. If the intent is to us...