uboot-imx

include: introduce include/linux/io.h as a wrapper of asm/io.h

In the latest Linux coding style, <linux/io.h> should be included
rather than <asm/io.h>.  To follow this standard also in U-Boot,
add include/linux/io.h.  Currently, it just includes <asm/io.h>.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>