Instead of having to duplicate the description of the properties needed
for interrupt support, reference the new standard document.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>
[Minor spelling correction]
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>