ARM: fix ioremap/iounmap for !CONFIG_MMU

With commit 4fe7ef3a (ARM: provide runtime hook for ioremap/iounmap),
compiles with !CONFIG_MMU were broken. Rename nommu __iounmap to
__arm_iounmap and add arch_ioremap_caller and arch_iounmap. Its
not expected that these need to be overriden for !CONFIG_MMU, so setting
the function ptrs has no effect in this case.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>