dt-bindings: Relocate Tegra20 memory controller bindings

Move the device tree bindings for the Tegra20 memory controller to the
same location as the Tegra30 (and later) memory controller bindings.

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