dt-bindings: tegra186-hsp: add type for shared mailboxes

Tegra234 supports sending/receiving 32-bit and 128-bit data over
a shared mailbox. Based on the data size to be used, clients need
to specify the type of shared mailbox in the device tree.

Add a macro for 128-bit shared mailbox. Mailbox clients can use this
macro as a flag in device tree to enable 128-bit data support for a
shared mailbox.

Signed-off-by: Kartik <kkartik@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>