devicetree: Add binding for Sigma Designs SMP86xx interrupt controller

This adds a binding for the secondary interrupt controller in
Sigma Designs SMP86xx and SMP87xx chips.

Signed-off-by: Mans Rullgard <mans@mansr.com>
[ jac: use 'interrupt-controller@XXX' notation in binding doc ]
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/1453313237-18570-1-git-send-email-mans@mansr.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>