irqchip/atmel-aic: Fix unbalanced refcount in aic_common_rtc_irq_fixup()

of_find_compatible_node() is calling of_node_put() on its first argument
thus leading to an unbalanced of_node_get/put() issue if the node has not
been retained before that.

Instead of passing the root node, pass NULL, which does exactly the same:
iterate over all DT nodes, starting from the root node.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Fixes: 3d61467f ("irqchip: atmel-aic: Implement RTC irq fixup")
Cc: <stable@vger.kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>