iommu/amd: Treat per-device exclusion ranges as r/w unity-mapped regions

Some buggy BIOSes might define multiple exclusion ranges of the
IVMD entries which are associated with the same IOMMU hardware.
This leads to the overwritten exclusion range (exclusion_start
and exclusion_length members) in set_device_exclusion_range().

Here is a real case:
When attaching two Broadcom RAID controllers to a server, the first
one reports the failure during booting (the disks connecting to the
RAID controller cannot be detected).

This patch prevents the issue by treating per-device exclusion
ranges as r/w unity-mapped regions.

Discussion:
  * https://lists.linuxfoundation.org/pipermail/iommu/2019-November/040140.html



Suggested-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>