ARM: ixp4xx: mark ixp4xx_irq_setup as __init

Kbuild complains about ixp4xx_irq_setup not being __init
itself in some configurations:

WARNING: vmlinux.o(.text+0x85bae4): Section mismatch in reference from the function ixp4xx_irq_setup() to the function .init.text:set_handle_irq()
The function ixp4xx_irq_setup() references
the function __init set_handle_irq().
This is often because ixp4xx_irq_setup lacks a __init
annotation or the annotation of set_handle_irq is wrong.

I suspect it normally gets inlined, so we get no such warning,
but clang makes this obvious when the function is left out
of line.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>