dt-bindings: tegra186-hsp: Add shared mailboxes

Shared mailboxes are a mechanism to transport data from one processor in
the system to another. They are bidirectional links with both a producer
and a consumer. Interrupts are used to let the consumer know when data
was written to the mailbox by the producer, and to let the producer know
when the consumer has read the data from the mailbox. These interrupts
are mapped to one or more "shared interrupts". Typically each processor
in the system owns one of these shared interrupts.

Add documentation to the device tree bindings about how clients can use
mailbox specifiers to request a specific shared mailbox and select which
direction they drive. Also document how to specify the shared interrupts
in addition to the existing doorbell interrupt.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>