of: Add simple panel device tree binding

This binding specifies a set of common properties for display panels. It
can be used as a basis by bindings for specific panels.

Bindings for three specific panels are provided to show how the simple
panel binding can be used.

Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>