dt-bindings: simple-framebuffer: Add requirement for pipelines

Both the allwinner and amlogic compatibles require that either the
allwinner,pipeline or the amlogic,pipeline property is set. This was
dropped during the conversion since we didn't have conditionals back then,
but we can express this properly now.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>