dt-bindings: reserved-memory: Support framebuffer reserved memory

Document the "framebuffer" compatible string for reserved memory nodes
to annotate reserved memory regions used for framebuffer carveouts.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-4-thierry.reding@gmail.com