dt-bindings: Clarify interrupts-extended usage

Reading the description about when to use interrupts-extended leads some
developers to think that it shouldn't be used unless a device has
interrupts from more than one interrupt controller. This isn't true. We
should encourage devicetree writers to use this property in situations
where it isn't the inherited interrupt-parent so that we have less
properties in a DT node by virtue of not having to specify an
interrupt-parent and an interrupts property.

Reported-by: Alexandru M Stan <amstan@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>