dt-bindings: ARM: tegra: Add Acer Iconia Tab A500

Add a binding for the Tegra20-based Acer Iconia Tab A500 tablet device.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>