irqchip: armada-370-xp: move IRQ handler to avoid forward declaration

If we move the IRQ handler function above the initialization function,
we avoid a forward declaration. This wasn't done as part of the
previous commit, in order to increase the readibility of the previous
commit, who was also moving the IRQ controller driver from arch/arm to
drivers/irqchip.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>