iommu/amd: Define bit fields for DTE particularly

In AMD-Vi spec several bits of IO PTE fields and DTE fields are similar
so that both of them can share the same MACRO definition. However
defining them respectively can make code more read-able. Do it now.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>