dt-bindings: Add schemas for simple-framebuffer

The simple framebuffer is a binding that allows the bootloader to setup a
framebuffer, describe it in the Device Tree for the OS to pick it up and
use it as is.

Replace the current binding by a schema to allow the validation tools to
check those nodes.

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>