x86, gart: fix gart detection for Fam11h CPUs

Impact: fix AMD Family 11h boot hangs / USB device problems

The AMD Fam11h CPUs have a K8 northbridge. This northbridge is different
from other family's because it lacks GART support (as I just learned).

But the kernel implicitly expects a GART if it finds an AMD northbridge.

Fix this by removing the Fam11h northbridge id from the scan list of K8
northbridges. This patch also changes the message in the GART driver
about missing K8 northbridges to tell that the GART is missing which is
the correct information in this case.

Reported-by: Jouni Malinen <jkmalinen@gmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>