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

arch/powerpc/configs/ppc64_defconfig

@@ -5,6 +5,7 @@ CONFIG_NO_HZ=y
 CONFIG_HIGH_RES_TIMERS=y
 CONFIG_BPF_SYSCALL=y
 CONFIG_BPF_JIT=y
+CONFIG_PREEMPT_VOLUNTARY=y
 CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
 CONFIG_TASKSTATS=y
 CONFIG_TASK_DELAY_ACCT=y