board: stm32mp1: enable simple framebuffer node for splashscreen

Enable an existing simple framebuffer node in the Linux kernel device
tree and the add the associated reserved memory node to preserved the
resources (clock, memory) used by the stm32 video driver to display
the splashscreen = background in exlinux.conf file.

These resources will be released by the Linux driver only when the
associated driver is ready to avoid transition issues during the Linux
kernel initialization between U-Boot splash screen and the final display.

See Linux documentation for details:
Documentation/devicetree/bindings/display/simple-framebuffer.yaml

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