[ARM] 3772/1: Fix compilation error in mach-ixp4xx/nslu2*

Patch from Martin Michlmayr

Include linux/irq.h in the nslu2 code in order to avoid the following
compiler error:

  CC      arch/arm/mach-ixp4xx/nslu2-power.o
arch/arm/mach-ixp4xx/nslu2-power.c: In function 'nslu2_power_init':
arch/arm/mach-ixp4xx/nslu2-power.c:53: warning: implicit declaration of function 'set_irq_type'
arch/arm/mach-ixp4xx/nslu2-power.c:53: error: 'IRQ_TYPE_LEVEL_LOW' undeclared (first use in this function)
arch/arm/mach-ixp4xx/nslu2-power.c:53: error: (Each undeclared identifier is reported only once
arch/arm/mach-ixp4xx/nslu2-power.c:53: error: for each function it appears in.)
arch/arm/mach-ixp4xx/nslu2-power.c:54: error: 'IRQ_TYPE_LEVEL_HIGH' undeclared (first use in this function)
make[5]: *** [arch/arm/mach-ixp4xx/nslu2-power.o] Error 1

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>