ASoC: atmel: simplify Kconfig

Enclose the options in if SND_ATMEL_SOC ... endif to remove the dependency.
Also remove the useless description for SND_ATMEL_SOC_SSC.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>