irqchip/gic-v3-its: Implement irq_set_irqchip_state for pending state

Allow the pending state of an LPI to be set or cleared via
irq_set_irqchip_state.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>