x86/iommu: Clean up the CONFIG_GART_IOMMU config option a bit

Improve the explanation of this config option.

Cc: Andi Kleen <ak@linux.intel.com>
Cc: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/n/tip-gUMmysvsbl3mccbyf6olmxqg@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>