dt-bindings: memory: Document Tegra210 EMC table

Add device tree bindings for the Tegra210 EMC table that is passed via a
reserved-memory device tree node.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217165919.2700920-1-thierry.reding@gmail.com