irqchip/irq-mvebu-icu: Make legacy_bindings static

Fix the following sparse warning:

drivers/irqchip/irq-mvebu-icu.c:69:1: warning: symbol 'legacy_bindings'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200417074046.46771-1-yanaijie@huawei.com