iommu/amd: Document which IRTE fields amd_iommu_update_ga() can modify

iommu/amd: Document which IRTE fields amd_iommu_update_ga() can modify

Add a comment to amd_iommu_update_ga() to document what fields it can
safely modify without issuing an invalidation of the IRTE, and to explain
its role in keeping GA IRTEs up-to-date.

Per page 93 of the IOMMU spec dated Feb 2025:

  When virtual interrupts are enabled by setting MMIO Offset 0018h[GAEn] and
  IRTE[GuestMode=1], IRTE[IsRun], IRTE[Destination], and if present IRTE[GATag],
  are not cached by the IOMMU. Modifications to these fields do not require an
  invalidation of the Interrupt Remapping Table.

Link: https://lore.kernel.org/all/9b7ceea3-8c47-4383-ad9c-1a9bbdc9044a@oracle.com
Cc: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20250611224604.313496-41-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>