hw/ppc/spapr_rng: Introduce CONFIG_SPAPR_RNG switch for spapr_rng.c

hw/ppc/spapr_rng: Introduce CONFIG_SPAPR_RNG switch for spapr_rng.c

The spapr-rng device is suboptimal when compared to virtio-rng, so
users might want to disable it in their builds. Thus let's introduce
a proper CONFIG switch to allow us to compile QEMU without this device.
The function spapr_rng_populate_dt is required for linking, so move it
to a different location.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>