Have the GART_IOMMU help text specify that this is the hardware IOMMU in
amd64 processors.  This will be significant if/when other IOMMUs are
added to the x86-64 architecture. :-)

Also, note that the previous help text stated that IOMMU was needed for
>3GB memory instead of >4GB.  This is fixed in the newer version.
Signed-off-by: Jon Mason <jdmason@us.ibm.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>