x86, amd_nb: Enable GART support for AMD family 0x15 CPUs

AMD CPU family 0x15 still supports GART for compatibility reasons.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
LKML-Reference: <20100930124316.GG20545@loge.amd.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>