irqchip/gic-v3-its: Factor out wait_for_syncr primitive

Waiting for a redistributor to have performed an operation is a
common thing to do, and the idiom is already spread around.
As we're going to make even more use of this, let's have a primitive
that does just that.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191027144234.8395-3-maz@kernel.org
Link: https://lore.kernel.org/r/20191108165805.3071-3-maz@kernel.org