dt-bindings: sram: Document NVIDIA Tegra SYSRAM

Tegra SoCs have extra on-chip RAM that can be used for inter-processor
communication. Tegra186 and later make use of it to establish a two-way
channel between the CCPLEX and BPMP. Add missing compatible strings for
Tegra186 and Tegra194 as well as a new compatible string for Tegra234.

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