ARM: dts: imx: update #interrupt-cells for gpio nodes

Change the value of #interrupt-cells of gpio nodes from 1 to 2, so that
the gpio irq type can be specified in device tree.  Also update binding
document to make it clear.

Reported-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>