dt-bindings: ARM: tegra: Document Pegatron Chagall

Document Pegatron Chagall, which is Tegra30-based tablet device.

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