ARM: tegra: Add nvidia,memory-controller phandle to Tegra20 EMC device-tree

Add nvidia,memory-controller to the Tegra20 External Memory Controller
node. This allows to perform a direct lookup of the Memory Controller
instead of walking up the whole tree. This puts Tegra20 device-tree on
par with Tegra30+.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>