dt-bindings: net: adin: Fix adi,phy-output-clock description syntax

"make dt_binding_check":

    Documentation/devicetree/bindings/net/adi,adin.yaml:40:77: [error] syntax error: mapping values are not allowed here (syntax)

The first line of the description ends with a colon, hence the block
needs to be marked with a "|".

Fixes: 1f77204e ("dt-bindings: net: adin: document phy clock output properties")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/6fcef2665a6cd86a021509a84c5956ec2efd93ed.1653401420.git.geert+renesas@glider.beSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/adi,adin.yaml

@@ -37,7 +37,8 @@ properties:
     default: 8
 
   adi,phy-output-clock:
-    description: Select clock output on GP_CLK pin. Two clocks are available:
+    description: |
+      Select clock output on GP_CLK pin. Two clocks are available:
       A 25MHz reference and a free-running 125MHz.
       The phy can alternatively automatically switch between the reference and
       the 125MHz clocks based on its internal state.