x86/vt-d: Fix documentation of DRHD

According to "Intel Virtualization Technology for Directed
I/O" specification, DRHD stands for "DMA Remapping Hardware
Unit Definition" , not "DMA Engine Reporting Structure".
Signed-off-by: Nan Xiao <nan.xiao@hp.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/Intel-IOMMU.txt

@@ -10,7 +10,7 @@ This guide gives a quick cheat sheet for some basic understanding.
 Some Keywords
 
 DMAR - DMA remapping
-DRHD - DMA Engine Reporting Structure
+DRHD - DMA Remapping Hardware Unit Definition
 RMRR - Reserved memory Region Reporting Structure
 ZLR  - Zero length reads from PCI devices
 IOVA - IO Virtual address.