dt-bindings: SLIMBus: add slim devices optional properties

This patch adds an optional SLIMBus Interface device phandle property
that could be used by some of the SLIMBus devices.

Interface device is mostly used with devices that are dealing
with streaming.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200109103148.5612-2-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/slimbus/bus.txt

@@ -32,6 +32,10 @@ Required property for SLIMbus child node if it is present:
 	 	  Product Code, shall be in lower case hexadecimal with leading
 		  zeroes suppressed
 
+Optional property for SLIMbus child node if it is present:
+- slim-ifc-dev	- Should be phandle to SLIMBus Interface device.
+		  Required for devices which deal with streams.
+
 SLIMbus example for Qualcomm's slimbus manager component:
 
 	slim@28080000 {
@@ -43,8 +47,14 @@ SLIMbus example for Qualcomm's slimbus manager component:
 		#address-cells = <2>;
 		#size-cell = <0>;
 
+		codec_ifd: ifd@0,0{
+			compatible = "slim217,60";
+			reg = <0 0>;
+		};
+
 		codec: wcd9310@1,0{
 			compatible = "slim217,60";
 			reg = <1 0>;
+			slim-ifc-dev  = <&codec_ifd>;
 		};
 	};