x86: add new cache descriptor

The latest rev of Intel doc AP-485 details a new cache
descriptor that we don't yet support.
A 6MB 24-way assoc L2 cache.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>