Add few corrections and fix issue for PowerPC

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@marius, is it correct now in your view?

Undochanges for sparc64, now things for sparc are as before.
Keep dev/ofw/ofw_pci.h to the existed scheme.
Move all of required declarations from dev/ofw/ofw_pci.h to new file dev/ofw/ofwpci.h.
Rename dev/ofw/ofw_pci.c file to dev/ofw/ofwpci.c. For now this file is not including to the sparc64 kernel, it uses only sparc64-specific ofw files.

Fix failures for sparc64 and powerpc architectures. OF PCI implementation from sparc64/ofw is now extracted. Anything that was machine-specific was moved to machine-specific file -
sparc64_ofw_pci.c (renamed from ofw_pci.c). The code which was identical or largely common went into general dev/ofw_pci.c file. Structure ofw_pci_softc is now the same for every architecture.