hw/ppc/Kconfig: NVDIMM is a hard requirement for the pseries machine

When building QEMU with "--without-default-devices", the pseries
machine fails to start even when running with the --nodefaults option:

 $ ./qemu-system-ppc64 --nodefaults -M pseries
 Type 'spapr-nvdimm' is missing its parent 'nvdimm'
 Aborted (core dumped)

Looks like NVDIMM is a hard requirement for this machine nowadays.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230504180521.220404-1-thuth@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/Kconfig

@@ -3,7 +3,7 @@ config PSERIES
     imply PCI_DEVICES
     imply TEST_DEVICES
     imply VIRTIO_VGA
-    imply NVDIMM
+    select NVDIMM
     select DIMM
     select PCI
     select SPAPR_VSCSI