dt-bindings: memory: tegra124: emc: Document new interconnect property

dt-bindings: memory: tegra124: emc: Document new interconnect property

External memory controller is interconnected with memory controller and
with external memory. Document new interconnect property which turns
External Memory Controller into interconnect provider.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201104164923.21238-14-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>