video: mbxfb: Include linux/io.h instead of asm/io.h

This fixes below build error:

  CC [M]  drivers/video/mbx/mbxfb.o
drivers/video/mbx/mbxfb.c: In function 'mbxfb_probe':
drivers/video/mbx/mbxfb.c:942:2: error: implicit declaration of function 'devm_ioremap_nocache' [-Werror=implicit-function-declaration]
drivers/video/mbx/mbxfb.c:942:22: warning: assignment makes pointer from integer without a cast [enabled by default]
drivers/video/mbx/mbxfb.c:952:21: warning: assignment makes pointer from integer without a cast [enabled by default]
cc1: some warnings being treated as errors

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>