ALSA: asoc: kbuild - only show menus for the current ASoC CPU platform.

We don't want to see ASoC platform menus for other non selected
architectures in our config.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>