irqchip/ti-sci-inta: Add support for INTA directly connecting to GIC

Driver assumes that Interrupt parent to Interrupt Aggregator is always
Interrupt router. This is not true always and GIC can be a parent to
Interrupt Aggregator. Update the driver to detect the parent and request
the parent irqs accordingly.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200806074826.24607-11-lokeshvutla@ti.com