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.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>