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>