Allow up to 24dB digital gain to be applied when using IQAudIO DAC

24db_digital_gain DT param can be used to specify that PCM512x
codec "Digital" volume control should not be limited to 0dB gain,
and if specified will allow the full 24dB gain.

Signed-off-by: DigitalDreamtime <clive.messer@digitaldreamtime.co.uk>