x86, AMD IOMMU: add documentation for kernel parameters

Add documentation for the kernel parameters introduced with this driver.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: Sebastian.Biemueller@amd.com
Cc: robert.richter@amd.com
Cc: joro@8bytes.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -271,6 +271,18 @@ and is between 256 and 4096 characters. It is defined in the file
 	aic79xx=	[HW,SCSI]
 			See Documentation/scsi/aic79xx.txt.
 
+	amd_iommu=	[HW,X86-84]
+			Pass parameters to the AMD IOMMU driver in the system.
+			Possible values are:
+			off     - disable the driver for AMD IOMMU
+			isolate - enable device isolation (each device, as far
+			          as possible, will get its own protection
+			          domain)
+	amd_iommu_size= [HW,X86-64]
+			Define the size of the aperture for the AMD IOMMU
+			driver. Possible values are:
+			'32M', '64M' (default), '128M', '256M', '512M', '1G'
+
 	amijoy.map=	[HW,JOY] Amiga joystick support
 			Map of devices attached to JOY0DAT and JOY1DAT
 			Format: <a>,<b>