hw/ppc: Give sam46ex its own config option

At present the Sam460ex board is activated by the general CONFIG_PPC4XX
option.  However that includes the board for both ppc-softmmu and
(deprecated) ppcemb-softmmu builds.  As Sam460ex is developed, that would
require adding more things into ppcemb-softmmu, which we don't want to do.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>