ASoC: img-spdif-in: Add runtime PM

Add runtime PM to disable the clock when the device is not in use.
The ASoC core takes care of the pm_runtime_get_sync / put calls so we
just need to provide runtime_suspend / runtime_resume functions and
enable runtime PM.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>