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 <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

···
 interrupt client node or in any of its parent nodes. Interrupts listed in the
 "interrupts" property are always in reference to the node's interrupt parent.
 
-The "interrupts-extended" property is a special form for use when a node needs
-to reference multiple interrupt parents. Each entry in this property contains
-both the parent phandle and the interrupt specifier. "interrupts-extended"
-should only be used when a device has multiple interrupt parents.
+The "interrupts-extended" property is a special form; useful when a node needs
+to reference multiple interrupt parents or a different interrupt parent than
+the inherited one. Each entry in this property contains both the parent phandle
+and the interrupt specifier.
 
   Example:
 	interrupts-extended = <&intc1 5 1>, <&intc2 1 0>;