dt-bindings: memory: Add Tegra186 memory subsystem

The NVIDIA Tegra186 SoC contains a memory subsystem composed of the
memory controller and the external memory controller. The memory
controller provides interfaces for the memory clients to access the
memory. Accesses can be either bounced through the SMMU for IOVA
translation or directly to the EMC.

The bulk of the programming of the external memory controller happens
through interfaces exposed by the BPMP. Describe this relationship by
adding a phandle reference to the BPMP to the EMC node.

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