ASoC: es7134: add dt-bindings for the es7134 dac

Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/everest,es7134.txt

+ES7134 i2s DA converter
+
+Required properties:
+- compatible : "everest,es7134" or "everest,es7144"
+
+Example:
+
+i2s_codec: external-codec {
+	compatible = "everest,es7134";
+};