video: fbdev: amifb: add FIXME about dead APUS support

On 5/14/20 10:21 PM, Geert Uytterhoeven wrote:

> These #ifdefs are relics from APUS (Amiga Power-Up System), which
> added a PPC board.  APUS support was killed off a long time ago,
> when arch/ppc/ was still king, but these #ifdefs were missed, because
> they didn't test for CONFIG_APUS.

Add FIXME about using the C code variants (APUS ones) in the future.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/767d36ff-22ec-8136-7ebc-1d9d0d3ac98d@samsung.com