ASoC: atmel-ssc: include linux/io.h for raw io

Include linux/io.h for raw io operations in atmel-scc header.

This fixes the following build error:
  CC [M]  sound/soc/atmel/atmel_ssc_dai.o
sound/soc/atmel/atmel_ssc_dai.c: In function 'atmel_ssc_interrupt':
sound/soc/atmel/atmel_ssc_dai.c:171: error: implicit declaration of function '__raw_readl'
sound/soc/atmel/atmel_ssc_dai.c: In function 'atmel_ssc_shutdown':
sound/soc/atmel/atmel_ssc_dai.c:249: error: implicit declaration of function '__raw_writel'

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>