arch,openrisc: Convert smp_mb__*()

Openrisc fully relies on asm-generic/barrier.h and therefore its
smp_mb() is barrier().

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-sxgxgqag9tond4kji07d22oh@git.kernel.org
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux@lists.openrisc.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>