The bcm2836_smp_boot_secondary() is not declared or used elsewhere
so make it static to fix the following warning:

drivers/irqchip/irq-bcm2836.c:227:12: warning: symbol 'bcm2836_smp_boot_secondary' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lkml.kernel.org/r/1465407697-8116-1-git-send-email-ben.dooks@codethink.co.ukSigned-off-by: Jason Cooper <jason@lakedaemon.net>