fbdev: sh_mobile_hdmi: Add select SND_SOC to Kconfig

This patch solve compile error when .config doesn't have
CONFIG_SND_SOC which is needed for HDMI sound.
It was reported by Guennadi as follows

Besides, I think, this will not link without CONFIG_SND_SOC.

Reported-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>