dt-bindings: mailbox: tegra: Document Tegra264 HSP

Add the compatible string for the HSP block found on the Tegra264 SoC.
The HSP block in Tegra264 is not register compatible with the one in
Tegra194 or Tegra234 hence there is no fallback compatibility string.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>