ARM: dts: AM33xx: Correct gpio #interrupt-cells property

Following commit ff5c9059

 and therefore other omap platforms using
the gpio-omap driver correct the #interrupt-cells property on am33xx
too. The omap gpio binding documentaion also states that
the #interrupt-cells property should be 2.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>