ARM: mxs: use debug_ll_io_init for low-level debug

The only user of the static mapping done in mx23_map_io and mx28_map_io
is low-level debug now.  Use debug_ll_io_init() instead, so that the
static mapping is used nowhere and can be removed completely later.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>