ARM: imx: remove last call to cpu_is_mx5*

The check for cpu_is_mx51/cpu_is_mx53() in mx51_revision()/mx53_revision()
is just a safety precaution, but there are only two callers of this
are using it only on the correct CPUs, and none of the other respective
functions have this extra check.

Removing these lets us kill off the cpu_is_* functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>