ASoC: img-spdif-out: Add suspend / resume handling

Implement suspend and resume callbacks to save and restore register
state, to support platforms where the power is disabled during suspend.

In future the driver could be converted to use regmap which would
simplify the suspend and resume code.

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