northbridge/amd/amdht: Add isochronous setup support

The coherent fabric on all Family 10h/15h devices supports
isochronous mode, which is required for IOMMU operation.

Add initial support for isochronous operation.

Change-Id: Idd7c9b94a65f856b0059e1d45f8719d9475771b6
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/12042
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
5 files changed