staging: mt7621-dts: add missing properties to gpio node

In order to let other devices reference the GPIO interrupts
if necessary properties 'interrupt-controller' and
'#interrupt-cells' becomes necessary. Add both of them to
complete gpio device tree node.

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