ARM: mvebu: fix compilation warning on Armada 370 (i.e. non-SMP)

The following appears during compilation for an Armada 370 target
because 'irq_controller_lock' is used only when CONFIG_SMP is
enabled:

 drivers/irqchip/irq-armada-370-xp.c:62:8: warning: 'irq_controller_lock' defined but not used [-Wunused-variable]

Fix that warning by moving declaration of 'irq_controller_lock'
inside existing #ifdef.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>