dt-bindings: arm: tegra: Document #reset-cells property of the Tegra20 MC

Memory Controller has a memory client "hot reset" functionality, which
resets the DMA interface of a memory client, so MC is a reset controller.

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