[POWERPC] Add function to check if address is an IO port

This adds a function that tells you if a given kernel virtual address
is hitting a PCI or ISA IO port permanent mapping or not. This is to
be used in the next patch to fix iomap APIs to properly unmap things.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>