irqchip: bcm2836: Move SMP startup code to arch/arm (v2)

In order to easily provide SMP for BCM2837 on 32-bit and 64-bit
the SMP startup code was placed in irq-bcm2836. That's not the
right approach. So move this code where it belongs.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 41f4988c ("irqchip/bcm2836: Add SMP support for the 2836")
Tested-by: Eric Anholt <eric@anholt.net>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>