ASoC: fsl_spdif: implement bypass mode from in to out

Implement SPDIF bypass mode. It implies internal SoC
routing of SPDIF input signal to SPDIF output signal. The
test bed requires two boards: B1 configured in bypass mode,
and B2 to feed B1 SPDIF RX port and read B1 SPDIF TX port:
   B2 TX -> B1 RX,
   B2 RX <- B1 TX.
The test procedure:
 a) Boot both boards
 b) B2: start "arecord <spdifcard> -r 48kHz | aplay <local DAC>"
 c) B2: start "aplay <spdifcard> -r 48kHz <2ch 48kHz audio file>"
 d) B1: enable bypass mode:
	amixer -cimxspdif cset numid=8,iface=PCM,name='Bypass Mode' on
 e) B2: check DAC audio, make sure the same sample rate is used at
	steps b) and c), in example above the rate is 48kHz.
 f) B1: try to run "aplay" or "arecord" on imxspdif card while in
	bypass mode - both must fail until bypass mode is disabled
 g) B1: disable bypass mode:
	amixer -cimxspdif cset numid=8,iface=PCM,name='Bypass Mode' off
 h) B1: check the usual playback and capture on imxspdif card.
	During this test try to set bypass mode - must not be allowed
	while playback or capture is running.

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1632649760-1651-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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