[ARM] MXC: add cpu_is_ macros

We had hardcoded cpu_is_ macros for mxc architectures till now. As we
want to run the same kernel on i.MX31 and i.MX35 this patch adds cpu_is_
macros which expand to 0 or 1 if only one architecture is compiled in and
only check for the cpu type if more than one architecture is compiled
in.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>