dt-bindings: memory: Add binding for NVIDIA Tegra30 External Memory Controller

Add device-tree binding for NVIDIA Tegra30 External Memory Controller.
The binding is based on the Tegra124 EMC binding since hardware is
similar, although there are couple significant differences.

Note that the memory timing description is given in a platform-specific
form because there is no detailed information on how to convert a
typical-common DDR timing into the register values. The timing format is
borrowed from downstream kernel, hence there is no hurdle in regards to
upstreaming of memory timings for the boards.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>