irqchip/al-fic: Add support for irq retrigger

Introduce interrupts retrigger support for Amazon's Annapurna Labs Fabric
Interrupt Controller.

Signed-off-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1568018358-18985-1-git-send-email-talel@amazon.com