irqchip/gic-v3: Configure SGIs as standard interrupts

Change the way we deal with GICv3 SGIs by turning them into proper
IRQs, and calling into the arch code to register the interrupt range
instead of a callback.

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