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 <antoniu.miclaus@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250127101026.5320-2-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -30,8 +30,9 @@ properties:
 
   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