x86, gart: add detection of AMD family 0x11 northbridges

This patch adds the detection of the northbridges in the AMD family 0x11
processors. It also fixes the magic numbers there while changing this code.

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