ARM: dts: imx6: update spdif sound card node properties

The merge of imx-spdif driver into fsl-asoc-card brought
new DT properties that can be used with the "fsl,imx-audio-spdif"
compatible:
* The "spdif-controller" property from imx-spdif is named "audio-cpu"
  in fsl-asoc-card.
* fsl-asoc-card uses codecs explicitly declared in DT
  with "audio-codec".
  With an S/PDIF, codec drivers spdif_transmitter and
  spdif_receiver should be used.
  Driver imx-spdif used instead the dummy codec and a pair of
  boolean properties, "spdif-in" and "spdif-out".

While backward compatibility is kept to support properties
"spdif-controller", "spdif-in" and "spdif-out", using new properties has
several benefits:
* "audio-cpu" and "audio-codec" are more generic names reflecting
  that the fsl-asoc-card driver supports multiple hardware.
  They are properties already used by devices using the
  fsl-asoc-card driver.
  They are also similar to properties of simple-card: "cpu" and "codec".
* "spdif-in" and "spdif-out" imply the use of the dummy codec in the
  driver. However, there are already two codec drivers for the S/PDIF,
  spdif_transmitter and spdif_receiver.
  It is better to declare S/PDIF Tx and Rx devices in a DT, and then
  reference them with "audio-codec" than using the dummy codec.

For those reasons, this commit updates in-tree DTs to use the new
properties:
* Rename "spdif-controller" property to "audio-cpu".
* Declare S/PDIF transmitter and/or receiver devices, and use them with
  the "audio-codec" property instead of "spdif-out" and/or "spdif-in".

These modifications were tested only on an imx8mn-evk board.

Note that out-of-tree and old DTs are still supported.

Signed-off-by: Elinor Montmasson <elinor.montmasson@savoirfairelinux.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>