dt-bindings: IDU-intc: Clean up documentation

* Some lines exceeded 80 characters.
* Clarified statement about AUX register interface

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