[ARM] nommu: add stubs for ioremap and friends

nommu doesn't have any form of remapping support, so ioremap, etc
become stubs which just return the casted address, doing nothing
else.

Move ioport_map(), ioport_unmap(), pci_iomap(), pci_iounmap()
into a separate file which is always built.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>