irqchip/bcm2836: Fix missing __init annotation

bcm2836_arm_irqchip_smp_init() calls set_smp_ipi_range(), which has
an __init annotation. Make sure the caller has the same annotation.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>