Rename ofwpci.c to ofw_pcib.c

It's a class0 driver that implements some pcib methods and creates
a pci bus as its children.
The "ofw_pci" name will be used by a new driver that will be a subclass
of the pci bus.
No functional changes intended.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30226