dt-bindings: display: simple-framebuffer: Support system memory framebuffers

In order to support framebuffers residing in system memory, allow the
memory-region property to override the framebuffer memory specification
in the "reg" property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-2-thierry.reding@gmail.com