ASoC: fsl_ssi: refine ipg clock usage in this module

Check if ipg clock is in clock-names property, then we can move the
ipg clock enable and disable operation to startup and shutdown, that
is only enable ipg clock when ssi is working and keep clock is disabled
when ssi is in idle.
But when the checking is failed, remain the clock control as before.

Tested-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>