ALSA: ASoC: Make OpenFirmware helper include file conditional

The OpenFirmware API headers don't build on all platforms so ensure
that they are not included unless they are being used.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>