dt: bindings: Move tegra20-emc binding to memory-controllers directory

Move tegra20-emc binding to the memory-controllers directory for
consistency with the other Tegra memory bindings.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>