powerpc/configs/64s: Enable CONFIG_MEM_SOFT_DIRTY

Enable CONFIG_MEM_SOFT_DIRTY to get some test coverage. Distros enable
it, and it has been broken previously. See commit 66b2ca08
("powerpc/64s/radix: Fix soft dirty tracking").
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231024112726.1819795-1-mpe@ellerman.id.au

arch/powerpc/configs/ppc64_defconfig

@@ -92,6 +92,7 @@ CONFIG_MEMORY_HOTPLUG=y
 CONFIG_MEMORY_HOTREMOVE=y
 CONFIG_KSM=y
 CONFIG_TRANSPARENT_HUGEPAGE=y
+CONFIG_MEM_SOFT_DIRTY=y
 CONFIG_ZONE_DEVICE=y
 CONFIG_NET=y
 CONFIG_PACKET=y