ASoC: simple-card: Add pm callbacks to platform driver

Set snd_soc_pm_ops for the pm ops to make sure that the ASoC level of PM
operations are going to happen. This is needed to get suspend/resume
working correctly when the audio is using simple-card.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>