ARM: ep93xx: clean-up mach/io.h

Move ep93xx specifics in mach/io.h to ep93xx-regs.h.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <rmallon@gmail.com>