Add a new function to activate an existing simple frame buffer node
and add the associated reserved memory, with no-map properties.

This device tree update is only done when the video device is active
and the video buffer is used.

This patch uses '#if CONFIG_IS_ENABLED(DM_VIDEO)' because
gd->video_bottom and gd->video_top are only defined when CONFIG_DM_VIDEO
is activated.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>