m68k/video: Create <asm/vga.h>

m68k/video: Create <asm/vga.h>

For now, it just contains the hack for cirrusfb on Amiga, which is moved
out of <video/vga.h> with some slight modifications (use raw_*() instead of
z_*(), which are defined on all m68k platforms).

This makes it safe to include <video/vga.h> in all contexts. Before it
could fail to compile with

include/video/vga.h: In function ‘vga_mm_r’:
include/video/vga.h:242: error: implicit declaration of function ‘z_readb’
include/video/vga.h: In function ‘vga_mm_w’:
include/video/vga.h:247: error: implicit declaration of function ‘z_writeb’
include/video/vga.h: In function ‘vga_mm_w_fast’:
include/video/vga.h:253: error: implicit declaration of function ‘z_writew’

or

include/video/vga.h:23:21: error: asm/vga.h: No such file or directory

depending on the value of CONFIG_AMIGA.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org