iommu/amd: Re-enable IOMMU event log interrupt after handling.

Current driver does not clear the IOMMU event log interrupt bit
in the IOMMU status register after processing an interrupt.
This causes the IOMMU hardware to generate event log interrupt only once.
This has been observed in both IOMMU v1 and V2 hardware.
This patch clears the bit by writing 1 to bit 1 of the IOMMU
status register (MMIO Offset 2020h)

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>