Now that dt-extract-example gained support for using root nodes
in examples, update the example for the simple-frambuffer binding to use it.

This gives us a better example and kill a long standing warning:

simple-framebuffer.example.dts:23.16-39.11:
Warning (chosen_node_is_root): /example-0/chosen: chosen node must be at root node

Note: To get the update dt-extract-example execute:
pip3 install git+https://github.com/devicetree-org/dt-schema.git@master --upgrade

v2:
  - fix spelling of framebuffer (Geert)
  - drop stdout-path (Rob)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200704143544.789345-2-sam@ravnborg.org