of: document external-memory-controller property in tegra124-car

This property contains a phandle to the EMC driver that is needed by the
EMC clock to request the EMC driver to do its part of the clock change
sequence.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>