ARM: orion5x: clean-up mach/io.h

Move orion5x specific mach/io.h parts into common.h.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Lennert Buytenhek <kernel@wantstofly.org>
Acked-by: Nicolas Pitre <nico@linaro.org>