dt-bindings: iio: adf4371: add refin mode

Add support for selecting between single-ended and differential
reference input.

Input frequency boundaries are change based on the mode selected
(single-ended/differential).

Signed-off-by: Antoniu Miclaus <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/frequency/adf4371.yaml

···
 
   clock-names:
     description:
-      Must be "clkin"
-    maxItems: 1
+      Must be "clkin" if the input reference is single ended or "clkin-diff"
+      if the input reference is differential.
+    enum: [clkin, clkin-diff]
 
   adi,mute-till-lock-en:
     type: boolean