irqchip/bcm2836: Drop smp_set_ops on arm64 builds

For arm64, the bootloader will instead be implementing the spin-table
enable method.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>