x86, amd: Extend AMD northbridge caching code to support "Link Control" devices

"Link Control" devices (NB function 4) will be used by L3 cache
partitioning on family 0x15.

Signed-off-by: Hans Rosenfeld <hans.rosenfeld@amd.com>
Cc: <andreas.herrmann3@amd.com>
LKML-Reference: <1295881543-572552-4-git-send-email-hans.rosenfeld@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>