ARM: imx: remove cpu_is_mx*()

The mxc_cpu_type and cpu_is_mx() logic is largely unused, and the
few remaining users were easy to convert into simpler code. Now that
they are gone, we can remove all those macros as well.

The related cpu_is_imx6*() set of function unfortunately is harder
to remove, so those are staying around for now.

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