ASoC: fsl_esai: Add new compatible string for imx6ull

Add new compatible string "fsl,imx6ull-esai" in the binding document.
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/1565346467-5769-2-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl,esai.txt

@@ -7,8 +7,11 @@ other DSPs. It has up to six transmitters and four receivers.
 
 Required properties:
 
-  - compatible		: Compatible list, must contain "fsl,imx35-esai" or
-			  "fsl,vf610-esai"
+  - compatible		: Compatible list, should contain one of the following
+			  compatibles:
+			  "fsl,imx35-esai",
+			  "fsl,vf610-esai",
+			  "fsl,imx6ull-esai",
 
   - reg			: Offset and length of the register set for the device.