amifb: Register Amiga-specific video modes with sysfs

Register the Amiga-specific video modes with sysfs, so you can see them in
/sys/class/graphics/fb0/modes and change the video mode by writing to
/sys/class/graphics/fb0/mode.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>