ARM: picoxcell: 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 picoxcell as well.

[ jiles: removed other, unused irq includes. ]

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>