powerpc: Fix sysfs pci mmap on 32-bit machines with 64-bit PCI

When manipulating 64-bit PCI addresses, the code would lose the
top 32-bit in a couple of places when shifting a pfn due to missing
type casting from the 32-bit pfn to a 64-bit resource before the
shift.

This breaks using newer X servers for example on 440 machines
with the PCI bus above 32-bit.

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