[ALSA] fix SND_SOC Kconfig

The new ALSA 'SOC' support has bogus Kconfig ... it should
not be presenting anything AT91-related except on AT91,
or anything PXA-related except on PXA.  Right now, x86
sees both of those menus, as do all other platforms.
This patch removes needless Kconfig layering, and the
related inappropriate choice presentation.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>