dt-bindings: serial: tegra-hsuart: Convert to json-schema

Convert the Tegra High-Speed UART bindings from the free-form text
format to json-schema.

While at it, also fix fix the example to reflect the correct compatible
string for Tegra30 chips.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>