When only a single i.MX SoC is selected the cpu_is_mx?() macros expand
to static 0 or 1. With the generic DT image enabled it could be though
that the SoC is not a i.MX at all, so make sure that with the generic
DT image enabled the cpu_is_mx?() macros always compare against
__imx_cpu_type

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