arm: add set_handle_irq() to register the parent IRQ controller handler function

In order to allow irqchip drivers to register their IRQ handling
function as the parent IRQ controller handler function, we provide a
convenience function. This will avoid poking directly into the global
handle_arch_irq variable.

Suggested by Arnd Bergmann.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Rob Herring: remove warning. 1st one to initialize wins.]
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Olof Johansson <olof@lixom.net>