Improve CONFIG_GART_IOMMU description and make it default y

Index: linux/arch/x86_64/Kconfig
===================================================================
--- linux.orig/arch/x86_64/Kconfig
+++ linux/arch/x86_64/Kconfig
@@ -329,12 +329,15 @@ config HPET_EMULATE_RTC
 
 config GART_IOMMU
 	bool "IOMMU support"
+	default y
 	depends on PCI
 	help
-	  Support the K8 IOMMU. Needed to run systems with more than 4GB of memory
+	  Support the IOMMU. Needed to run systems with more than 3GB of memory
 	  properly with 32-bit PCI devices that do not support DAC (Double Address
 	  Cycle). The IOMMU can be turned off at runtime with the iommu=off parameter.
 	  Normally the kernel will take the right choice by itself.
+	  This option includes a driver for the AMD Opteron/Athlon64 IOMMU
+	  and a software emulation used on some other systems.
 	  If unsure, say Y.
 
 # need this always enabled with GART_IOMMU for the VIA workaround