ASoC: dt-bindings: fsl_xcvr: Add document for XCVR

XCVR (Audio Transceiver) is a new IP module found on i.MX8MP.

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201013121733.83684-3-viorel.suman@oss.nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>