x86, cacheinfo: Cleanup L3 cache index disable support

Adaptions to the changes of the AMD northbridge caching code: instead
of a bool in each l3 struct, use a flag in amd_northbridges.flags to
indicate L3 cache index disable support; use a pointer to the whole
northbridge instead of the misc device in the l3 struct; simplify the
initialisation; dynamically generate sysfs attribute array.

Signed-off-by: Hans Rosenfeld <hans.rosenfeld@amd.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>