ASoC: dt-bindings: fsl_easrc: Add document for EASRC

EASRC (Enhanced Asynchronous Sample Rate Converter) is a new
IP module found on i.MX8MN.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/3195cad960113a089d63c10e2268d63b253a69c5.1587038908.git.shengjiu.wang@nxp.com


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