powerpc: Implement OF PCI address accessors stubs for CONFIG_PCI=n

To avoid "#ifdef CONFIG_PCI" in the drivers we should provide stubs in
place of OF PCI address accessors.

Without these stubs build breaks for drivers not strictly requiring PCI,
for example CONFIG_FB_OF=y without CONFIG_PCI:

  LD      .tmp_vmlinux1
drivers/built-in.o: In function `offb_map_reg':
offb.c:(.text+0x6e7c): undefined reference to `of_get_pci_address'

OF PCI IRQ accessors require pci_dev argument, so drivers using PCI
IRQs should depend on CONFIG_PCI anyway.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>