ASoC: atmel_ssc_dai: make option to choose clock

When SSC works in slave mode, according to the hardware design, the
clock can get from TK pin, also can get from RK pin. So, add one
parameter to choose where the clock from.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>