ARM: mxs: remove the .map_io declaration

Now that the ARM core code calls debug_ll_io_init, we can remove it from
the machine_desc declaration.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>