dt-bindings: mailbox: Add Tegra HSP binding

Add DT binding for the Hardware Synchronization Primitives (HSP). The
HSP is designed for the processors to share resources and communicate
with one another. A set of hardware synchronization primitives for
interprocessor communication (IPC) is provided. IPC protocols can use
use these hardware synchronization primitives when operating between
processors in an AMP configuration.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>