irqchip/ti-sci-intr: Add support for INTR being a parent to INTR

Driver assumes that Interrupt parent to Interrupt router is always GIC.
This is not true always and an Interrupt Router can be a parent to
Interrupt Router. 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/20200806074826.24607-7-lokeshvutla@ti.com