Add IQaudIO Sound Card support for Raspberry Pi

Set a limit of 0dB on Digital Volume Control

The main volume control in the PCM512x DAC has a range up to
+24dB. This is dangerously loud and can potentially cause massive
clipping in the output stages. Therefore this sets a sensible
limit of 0dB for this control.

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.

Modify IQAudIO DAC+ ASoC driver to set card/dai config from dt

Add the ability to set the card name, dai name and dai stream name, from
dt config.

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

IQaudIO: auto-mute for AMP+ and DigiAMP+

IQAudIO amplifier mute via GPIO22. Add dt params for "one-shot" unmute
and auto mute.

Revision 2, auto mute implementing HiassofT suggestion to mute/unmute
using set_bias_level, rather than startup/shutdown....
"By default DAPM waits 5 seconds (pmdown_time) before shutting down
playback streams so a close/stop immediately followed by open/start
doesn't trigger an amp mute+unmute."

Tested on both AMP+ (via DAC+) and DigiAMP+, with both options...

dtoverlay=iqaudio-dacplus,unmute_amp
 "one-shot" unmute when kernel module loads.

dtoverlay=iqaudio-dacplus,auto_mute_amp
 Unmute amp when ALSA device opened by a client. Mute, with 5 second delay
 when ALSA device closed. (Re-opening the device within the 5 second close
 window, will cancel mute.)

Revision 4, using gpiod.

Revision 5, clean-up formatting before adding mute code.
 - Convert tab plus 4 space formatting to 2x tab
 - Remove '// NOT USED' commented code

Revision 6, don't attempt to "one-shot" unmute amp, unless card is
successfully registered.

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