fbdev: atafb: Fix broken frame buffer after kexec

On Falcon, Atari frame buffer initialization relies on preprogrammed
register values.  These register values are changed to blank the
console.

Hence when using kexec to boot into a new kernel while the console is
blanked, the new kernel cannot determine the current video
configuration, and the Atari frame buffer driver fails to initialize:

    atafb: phys_screen_base 6ce000 screen_len 4096

Fix this by doing a straight-forward conversion of the driver to a
platform device driver, and adding a shutdown handler that unblanks the
display.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>