powerpc/powernv: Setup PE for root bus

There is no parent bridge for root bus, meaning pcibios_setup_bridge()
isn't invoked for root bus. The PE for root bus is the ancestor of
other PEs in PELTV. It means we need PE for root bus populated before
all others.

This populates the PE for root bus in pcibios_setup_bridge() path
if it's not populated yet. The PE number next to the reserved one
is used as the PE# to avoid holes in continuous M64 space.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>