dt-bindings: simple-framebuffer: allow standalone compatible

Qualcomm platforms use "simple-framebuffer" without a more specific
compatible. Allow that in the binding doc.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>