[ALSA] Add SUPERH depends to sound/soc/sh/Kconfig

Currently you will see an empty "SoC Audio support for SuperH" menu
when building for other archs (example pxa).
This patch adds "depends on SUPERH" to remove that empty menu.

Signed-off-by: Kristoffer Ericson <kristoffer.ericson@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>