x86: nmi: Add Intel processor 0x6f4 to NMI perfctr1 workaround

Expand Intel NMI perfctr1 workaround to include a Core2 processor stepping
(cpuid family-6, model-f, stepping-4).  Resolves a situation where the NMI
would not enable on these processors.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: prarit@redhat.com
Cc: suresh.b.siddha@intel.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>