pseries: Check for duplicate addresses on the spapr-vio bus

Check that devices on the spapr vio bus aren't given duplicate
addresses. Currently we will not run with duplicate devices, the
fdt code will spot it, but the error reporting is not great. With
this patch we can report the error nicely in terms of the device
names given by the user.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
