ASoC: davinci-mcasp: add support for suspend and resume

When the system returns from suspend, it looses its configuration. Most
of it is restored by running a normal audio stream startup, but the DAI
format is left unset as that's configured on the audio device creation.

Hence, it suffices here to care for the registers which are touched by
davinci_mcasp_set_dai_fmt() and restore them when the system is resumed.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>