ASoC: nau8824: user configuration of key detection

The SAR ADC of key press detection varies depending on headset.
We can't make a set of common threshold values for every case.
Therefore, the driver provides configuration for user and
they can set up values by UCM configuration.

Signed-off-by: John Hsu <KCHSU0@nuvoton.com>
Signed-off-by: Mark Brown <broonie@kernel.org>