staging: mt7621-dts: add interrupt device tree nodes for the gpio controller

The GPIO controller of mt7621 can receive interrupts on any
of the GPIOs, either edge or level. It then interrupts the CPU using
GIC INT12. Update device tree accordly.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>