ARM: mxs: Remove init_irq declaration in machine description

Commit ebafed7a ("ARM: irq: Call irqchip_init if no init_irq function is
specified") removed the need to explictly setup the init_irq field in
the machine description when using only irqchip_init. Remove that
declaration for mxs as well.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>