x86/amd-iommu: Change alloc_pte to support 64 bit address space

This patch changes the alloc_pte function to be able to map
pages into the whole 64 bit address space supported by AMD
IOMMU hardware from the old limit of 2**39 bytes.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>