irqchip/bcm2836: Use a more generic memory barrier call

dsb() requires an argument on arm64, so we needed to add "sy".
Instead, take this opportunity to switch to the same smp_wmb() call
that gic uses for its IPIs.  This is a less strong barrier than we
were doing before (dmb(ishst) compared to dsb(sy)), but it seems to be
the correct one.

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