irqchip/gic-v3-its: Add its_vlpi_map helpers

Obtaining the mapping information for a VLPI is something quite common,
and the GICv4.1 code is going to make even more use of it. Expose it as
a separate set of helpers.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191027144234.8395-8-maz@kernel.org
Link: https://lore.kernel.org/r/20191108165805.3071-8-maz@kernel.org