dt-bindings: dw-apb-ictl: Update binding to describe use as primary interrupt controller

Add the required updates to describe the use of dw-apb-ictl as a primary
interrupt controller.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200924071754.4509-5-thunder.leizhen@huawei.com