powerpc: Fix build without CONFIG_PCI

Commit fea80311
"iomap: make IOPORT/PCI mapping functions conditional"

Broke powerpc build without CONFIG_PCI as we would still define
pci_iomap(), which overlaps with the new empty inline in the headers.

Make our implementation conditional on CONFIG_PCI

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>