ARM: dts: aspeed: Add #interrupt-cells property to gpio controllers

Allows the GPIO controller to be used as an interrupt parent.

of_irq_find_parent() skips interrupt controller nodes that do
not have the #interrupt-cells property.

Signed-off-by: Mark Walton <mark.walton@serialtek.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>