irqchip/gic-v4.1: Plumb mask/unmask SGI callbacks

Implement mask/unmask for virtual SGIs by calling into the
configuration helper.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20200304203330.4967-11-maz@kernel.org