powerpcspe: fix PCI enumeration on ppce500

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

Reviewed by: jhibbits, bdragon
MFC after: 2 days
Sponsored by: Institudo de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D34621