x86/iommu/docs: Update AMD IOMMU specification document URL

The previous link to the AMD IOMMU specification is no longer available,
replace it with the latest one.

Signed-off-by: Jianlin Li <ljianlin99@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230923071344.540894-1-ljianlin99@gmail.com