dt-bindings: display: simple-framebuffer: Document 32-bit BGR format

This is a variant of the 32-bit RGB format where the red and blue
components are swapped.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-3-thierry.reding@gmail.com