powerpc: Fix domain numbers in /proc on 64-bit

The code to properly expose domain numbers in /proc is somewhat
bogus on ppc64 as it depends on the "buid" field being non-0,
but that field is really pseries specific.

This removes that code and makes ppc64 use the same code as 32-bit
which effectively decides whether to expose domains based on
ppc_pci_flags set by the platform, and sets the default for 64-bit
to enable domains and enable compatibility for domain 0 (which
strips the domain number for domain 0 to help with X servers).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>