Make the visibility of the tristate conditional on having the OpenFirmware
helper code enabed so that users who can't use it don't see the visible
option. Kconfig ignores dependencies for select so other users are
unaffected.

Thanks to Takashi for the suggestion.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>