video: mark nuc900fb_map_video_memory as __devinit

nuc900fb_map_video_memory is called by an devinit function
that may be called at run-time, but the function itself is
marked __init and will be discarded after boot.

To avoid calling into a function that may have been overwritten,
mark nuc900fb_map_video_memory itself as __devinit.

Without this patch, building nuc950_defconfig results in:

WARNING: drivers/video/built-in.o(.devinit.text+0x26c): Section mismatch in reference from the function nuc900fb_probe() to the function .init.text:nuc900fb_map_video_memory()
The function __devinit nuc900fb_probe() references
a function __init nuc900fb_map_video_memory().
If nuc900fb_map_video_memory is only used by nuc900fb_probe then
annotate nuc900fb_map_video_memory with a matching annotation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>