Extract common code from PowerPC's ofw_pci

Import portions of the PowerPC OF PCI implementation into
new file "ofw_pci.c", common for other platforms. The files ofw_pci.c and
ofw_pci.h from sys/powerpc/ofw no longer exist. All required declarations
are moved to sys/dev/ofw/ofw_pci.h.

This creates a new ofw_pci_write_ivar() function and modifies
ofw_pci_nranges(), ofw_pci_read_ivar(), ofw_pci_route_interrupt() methods.
Most functions contain existing ppc implementations in the majority
unchanged. Now there is no need to have multiple identical copies
of methods for various architectures.

Submitted by: Marcin Mazurek <mma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Annapurna Labs
Reviewed by: jhibbits, mmel
Differential Revision: https://reviews.freebsd.org/D4879