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>