hw/ppc/Kconfig: Let the Xilinx Virtex5 ML507 use the PPC-440 devices

When configured with --without-default-devices, the build fails:

    LINK    ppc-softmmu/qemu-system-ppc
  /usr/bin/ld: hw/ppc/virtex_ml507.o: in function `ppc440_init_xilinx':
  hw/ppc/virtex_ml507.c:112: undefined reference to `ppcuic_init'
  collect2: error: ld returned 1 exit status
  make[1]: *** [Makefile:206: qemu-system-ppc] Error 1
  make: *** [Makefile:483: ppc-softmmu/all] Error 2

Fix by selecting the PPC4XX config.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191231183216.6781-9-philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/ppc/Kconfig

@@ -110,6 +110,7 @@ config E500
 
 config VIRTEX
     bool
+    select PPC4XX
     select PFLASH_CFI01
     select SERIAL
     select XILINX