ARM: irq: Call irqchip_init if no init_irq function is specified

More and more sub-architectures are using only the irqchip_init
function. Make the core code call this function if no init_irq field is
provided in the machine description to remove some boilerplate code.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>