staging: mt7621-gpio: update #interrupt-cells for the gpio node

Most gpio chips have two cells for interrupts and this should be also.
Set this property in the device tree accordly fixing this up. In order
to make this working properly the xlate function for the irq_domain must
be updated to use the  'irq_domain_xlate_twocell' one in the driver.
One more minimal change is needed two refer gpio's interrupt-parent from
other nodes which is to add new 'gpio' label in the device tree.

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