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

Documentation/arch/x86/iommu.rst

@@ -5,7 +5,7 @@ x86 IOMMU Support
 The architecture specs can be obtained from the below locations.
 
 - Intel: http://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/vt-directed-io-spec.pdf
-- AMD: https://www.amd.com/system/files/TechDocs/48882_IOMMU.pdf
+- AMD: https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/specifications/48882_3_07_PUB.pdf
 
 This guide gives a quick cheat sheet for some basic understanding.