PPC: E500plat: Make a lot of PCI slots available

The ppce500 machine doesn't have to stick to hardware limitations,
as it's defined as being fully device tree based.

Thus we can change the initial PCI slot ID to 0x1 which gives us a
whopping 31 PCI devices we can support with this machine now!

Signed-off-by: Alexander Graf <agraf@suse.de>
