iommu/vt-d: Make Intel SVM code 64-bit only

Current Intel SVM is designed by setting the pgd_t of the processor page
table to FLPTR field of the PASID entry. The first level translation only
supports 4 and 5 level paging structures, hence it's infeasible for the
IOMMU to share a processor's page table when it's running in 32-bit mode.
Let's disable 32bit support for now and claim support only when all the
missing pieces are ready in the future.

Fixes: 1c4f88b7

 ("iommu/vt-d: Shared virtual address in scalable mode")
Suggested-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20200622231345.29722-2-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>