ARM: versatile: remove init_irq hook for DT boot

Now that versatile's irqchips are initialized from DT, the init_irq hook
can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>