dt-bindings: ARM: tegra: Add ASUS Google Nexus 7

Add a binding for the Tegra30-based ASUS Google Nexus 7 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>