AMD/iommu: SR56x0 Erratum 64 - Reset all head & tail pointers

AMD/iommu: SR56x0 Erratum 64 - Reset all head & tail pointers

Reference at time of patch:
http://support.amd.com/us/ChipsetMotherboard_TechDocs/46303.pdf

Erratum 64 states that the head and tail pointers for the Command buffer and
Event log are only reset on a cold boot, not a warm boot.

While the erratum is limited to systems using SR56xx chipsets (such as Family
10h CPUs), resetting the pointers is a sensible action in all cases, including
the PPR log for consistency.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>