ARM: imx: Remove unneeded .map_io initialization

If machine_desc.map_io is not set, devicemaps_init() in the common ARM
code will call debug_ll_io_init().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>