dt-bindings: dp-connector: Drop maxItems from -supply

The meta-schema recently gained a definition for the common -supply$
property, which denotes that maxItems is not a valid property. Drop this
to clear up the binding validation error.

Fixes: a46c1125

 ("dt-bindings: dp-connector: add binding for DisplayPort connector")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120175922.1579835-1-bjorn.andersson@linaro.org