MIPI-DSI devices, if they are controlled through the bus itself, have to
be described as a child node of the controller they are attached to.

Thus, there's no requirement on the controller having an OF-Graph output
port to model the data stream: it's assumed that it would go from the
parent to the child.

However, some bridges controlled through the DSI bus still require an
input OF-Graph port, thus requiring a controller with an OF-Graph output
port. This prevents those bridges from being used with the controllers
that do not have one without any particular reason to.

Let's drop that requirement.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220323154823.839469-1-maxime@cerno.tech