irqchip/gic: Convert to handle_strict_flow_irq()

Now that the proper infrastructure is in place, convert the irq-gic chip to
use handle_strict_flow_irq() along with IRQCHIP_AUTOMASKS_FLOW.

For EOImode=1, the Priority Drop is moved from gic_handle_irq() into
chip->irq_ack(). This effectively pushes the EOI write down into
->handle_irq(), but doesn't change its ordering wrt the irqaction
handling.

The EOImode=1 irqchip also gains IRQCHIP_EOI_THREADED, which allows the
->irq_eoi() call to be deferred to the tail of ONESHOT IRQ threads. This
means a threaded ONESHOT IRQ can now be handled entirely without a single
chip->irq_mask() call.

EOImode=0 handling remains unchanged.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>