irqchip/jcore-aic: Kill use of irq_create_strict_mappings()

irq_create_strict_mappings() is a poor way to allow the use of
a linear IRQ domain as a legacy one. Let's be upfront about it.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210406093557.1073423-4-maz@kernel.org