irqchip/git-v3-its: Implement irq_retrigger callback for device-triggered LPIs

It is pretty easy to provide a retrigger callback for the ITS,
as it we already have the required support in terms of
irq_set_irqchip_state().

Note that this only works for device-generated LPIs, and not
the GICv4 doorbells, which should never have to be retriggered
anyway.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>