ASoC: rt1308: add silence detection and manual PDB control

We enable the silence detection function in initial settings.
PDB control changes to manual mode, hence the driver could
fully control the AMP output on/off.

Signed-off-by: Shuming Fan <shumingf@realtek.com>
Link: https://lore.kernel.org/r/20190719063249.18806-1-shumingf@realtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>