dt-bindings: i2c: mux: i2c-demux-pinctrl: Define "i2c-parent" constraints

The 'phandle-array' type is a bit ambiguous. It can be either just an
array of phandles or an array of phandles plus args. "i2c-parent" is
the former and needs to constrain each entry to a single phandle value.
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240124190552.1551929-2-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/i2c/i2c-demux-pinctrl.yaml

@@ -40,6 +40,8 @@ properties:
 
   i2c-parent:
     $ref: /schemas/types.yaml#/definitions/phandle-array
+    items:
+      maxItems: 1
     description:
       List of phandles of I2C masters available for selection.  The first one
       will be used as default.