powerpcspe: fix PCI enumeration on ppce500

[alfredo]

Summary

This fixes PCI devices not being found on QEMU ppce500. This
generic board used to have its first PCI slot at 0x11, like the
mpc8544dsi and some real HW. After commit [1], it was changed to
0x1 and our driver wasn't prepared for that.

[1] https://gitlab.com/qemu-project/qemu/-/commit/3bb7e02a9725a24e5bf915b35f914f82f5b07a1f

Sponsored by: Institudo de Pesquisas Eldorado (eldorado.org.br)

Test Plan

Boot/run test on ppce500 and mpc8544ds under QEMU 6.2.0

[alfredo]

style

[bdragon]

Looks good to me on inspection. That was always something annoying about trying to configure the PCI bus on ppce500 in qemu. Things should be a lot easier to work with now, before this, we had to manually assign slots high enough.