x86: clean up amd_iommu documentation

amd_iommu=off was replaced with a common parameter, iommu=off.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: joerg.roedel@amd.com
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Cc: rjw@sisk.pl
Signed-off-by: Ingo Molnar <mingo@elte.hu>