ASoC: kirkwood: fix loss of external clock at probe time

At probe time, when the clock driver is not yet initialized, the
external clock of the kirkwood sound device will not be usable.

This patch fixes this problem defering the device probe.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mark Brown <broonie@linaro.org>