Binding: atmel-ssc: add option to choose clock

Add the option to choose clock on which pin input to SSC (as slave).
Default is on TK pin to SSC, add "atmel,clk-from-rk-pin" option to
specify the clock is on RK pin to SSC.

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>