ASoC: atmel-ssc: add pinctrl selection to driver

Add default pinctrl selection to atmel-ssc driver. The pinctrl
is mandatory.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
[nicolas.ferre@atmel.com: split dtsi and driver changes]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>