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>

drivers/irqchip/irq-ixp4xx.c

@@ -252,7 +252,7 @@ static const struct ixp4xx_irq_chunk ixp4xx_irq_chunks[] = {
  * @fwnode: Corresponding fwnode abstraction for this controller
  * @is_356: if this is an IXP43x, IXP45x or IXP46x SoC variant
  */
-static int ixp4xx_irq_setup(struct ixp4xx_irq *ixi,
+static int __init ixp4xx_irq_setup(struct ixp4xx_irq *ixi,
 				   void __iomem *irqbase,
 				   struct fwnode_handle *fwnode,
 				   bool is_356)