ARM: zynq: DT: Add interrupt-controller property to GPIO

GPIO can be used as interrupt-controller. Add the missing properties to
the GPIO node.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Reviewed-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>