dt-bindings: IDU-intc: Add support for edge-triggered interrupts

This updates the documentation for supporting an optional extra interrupt
cell to specify edge vs level triggered.

Signed-off-by: Mischa Jonker <mischa.jonker@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>