spapr_pci: make index property mandatory

PHBs can be created with an index property, in which case the machine
code automatically sets all the MMIO windows at addresses derived from
the index. Alternatively, they can be manually created without index,
but the user has to provide addresses for all MMIO windows.

The non-index way happens to be more trouble than it's worth: it's
difficult to use, keeps requiring (potentially incompatible) changes
when some new parameter needs adding, and is awkward to check for
collisions. It currently even has a bug that prevents to use two
non-index PHBs because their child DRCs are all derived from the
same index == -1 value, and, thus, collide.

This patch hence makes the index property mandatory. As a consequence,
the PHB's memory regions and BUID are now always configured according
to the index, and it is no longer possible to set them from the command
line.

This DOES BREAK backwards compat, but we don't think the non-index
PHB feature was used in practice (at least libvirt doesn't) and the
simplification is worth it.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>