amd-iommu: use a bitfield for DTE

amd-iommu: use a bitfield for DTE

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_dte) and uses this
structure in all DTE manipulation, resulting in much more readable and
compact code.

NOTE: This patch also includes some clean-up of get_dma_requestor_id() to
      change the types of the arguments from u16 to uint16_t.

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