fbdev: sh7760fb: allow modular build

There is no reason to prohibit sh7760fb from being built as a
loadable module as suggested by Geert, so change the config symbol
from bool to tristate to allow that and change the FB dependency as
needed.

Fixes: f75f71b2 ("fbdev/sh7760fb: Depend on FB=y")
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>