powerpc/configs/64s: Enable PREEMPT_VOLUNTARY

Traditionally on powerpc servers PREEMPT_NONE was used, but these days
multiple distros are building with PREEMPT_VOLUNTARY - Ubuntu, Fedora &
CentOS all enable it.

So update the upstream config to reflect that, and get test coverage
before code hits the distros.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-8-mpe@ellerman.id.au