ASoC: atmel: Add slave mode support to SSC in DSP Mode A

Add previously unsupported slave mode to the SSC peripheral when using
DSP/PCM Mode A format on the Atmel ARM platform.

Signed-off-by: Zoltan Puskas <zpuskas@dension.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>