The following build warning is seen with W=1:

warning: no previous prototype for ‘mx31lite_map_io’
[-Wmissing-prototypes] void __init mx31lite_map_io(void)

This function is only used in this file so make it "static".
Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>