irqchip/bcm7038-l1: Hide cpu offline callback when building for !SMP

When compiling bmips with SMP disabled, the build fails with:

drivers/irqchip/irq-bcm7038-l1.o: In function `bcm7038_l1_cpu_offline':
drivers/irqchip/irq-bcm7038-l1.c:242: undefined reference to `irq_set_affinity_locked'
make[5]: *** [vmlinux] Error 1

Fix this by adding and setting bcm7038_l1_cpu_offline only when actually
compiling for SMP. It wouldn't have been used anyway, as it requires
CPU_HOTPLUG, which in turn requires SMP.

Fixes: 34c53579

 ("irqchip/bcm7038-l1: Implement irq_cpu_offline() callback")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>