dt-bindings: display: simple-framebuffer: Document the panel node

Document the new panel node and what it is used for.

Signed-off-by: Rayyan Ansari <rayyan@ansari.sh>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230126182435.70544-3-rayyan@ansari.sh