dt-binding: display: Allow a single port node on rocktech, jh057n00900

The display has one port. Allow it in the binding.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200703114717.2140832-3-megous@megous.com