dt: bindings: tegra20-emc: Document interrupt property

EMC has a dedicated interrupt that is used to notify about completion of
HW operations. Document the interrupt property.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>