ARM: remove compile time __arch_ioremap/__arch_iounmap

Now that all custom ioremap/iounmap users are converted to runtime hook,
remove the compile time defines.

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