dt-bindings: irqchip: ti, sci-inta: Update docs to support different parent.

Drop the firmware related interrupt ranges and use the hardware specified
interrupt numbers within Interrupt Aggregator. This ensures interrupt
aggregator DT node need not assume any interrupt parent type.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200806074826.24607-8-lokeshvutla@ti.com