PowerTune controls the power level of the chip.  On playback this
indirectly controls things like the gain of the various output
amplifiers.  This can allow for the decrease of output levels
from the codec.  This adds controls for those power levels to
the driver.
Signed-off-by: Annaliese McDermond <nh6z@nh6z.net>
Signed-off-by: Mark Brown <broonie@kernel.org>