oprofile: Disable oprofile NMI timer on ppc64

We want to enable the hard lockup detector on ppc64, but right now
that enables the oprofile NMI timer too.

We'd prefer not to enable the oprofile NMI timer, it adds another
element to our PMU testing and it requires us to increase our
exported symbols (eg cpu_khz).

Modify the config entry for OPROFILE_NMI_TIMER to disable it on PPC64.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Robert Richter <rric@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>