amd-iommu: use a bitfield for PTE/PDE

amd-iommu: use a bitfield for PTE/PDE

The current use of get/set_field_from/in_reg_u32() is both inefficient and
requires some ugly casting.

This patch defines a new bitfield structure (amd_iommu_pte) and uses this
structure in all PTE/PDE manipulation, resulting in much more readable
and compact code.

NOTE: This commit also fixes one malformed comment in
      set_iommu_pte_present().

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>