irqchip/realtek-rtl: use irq_domain_add_linear()

When using an offset of 0, irq_domain_add_simple() is identical to
irq_domain_add_linear() on DT-based systems, so use the latter instead.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/0c4cd9f7661a30a4cb7ab9881c4a94bc8a379162.1663617425.git.sander@svanheule.net