x86: fix nmi_watchdog=2 on Pentium-D CPUs

implement nmi_watchdog=2 on this class of CPUs:

  cpu family      : 15
  model           : 6
  model name      : Intel(R) Pentium(R) D CPU 3.00GHz

the watchdog's ->setup() method is safe anyway, so if the CPU
cannot support it we'll bail out safely.

Signed-off-by: Ingo Molnar <mingo@elte.hu>