of/irq: Fix lookup to use 'interrupts-extended' property first

In case the Device Tree blob passed by the boot agent supplies both an
'interrupts-extended' and an 'interrupts' property in order to allow for
older kernels to be usable, prefer the new-style 'interrupts-extended'
property which conveys a lot more information.

This allows us to have bootloaders willingly maintaining backwards
compatibility with older kernels without entirely deprecating the
'interrupts' property.

Update the bindings documentation to describe a situation where both the
'interrupts-extended' and the 'interrupts' property are present, and
which one takes precedence over the other.

Cc: stable@vger.kernel.org # 3.13+
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

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

@@ -4,11 +4,13 @@ Specifying interrupt information for devices
 1) Interrupt client nodes
 -------------------------
 
-Nodes that describe devices which generate interrupts must contain an either an
-"interrupts" property or an "interrupts-extended" property. These properties
-contain a list of interrupt specifiers, one per output interrupt. The format of
-the interrupt specifier is determined by the interrupt controller to which the
-interrupts are routed; see section 2 below for details.
+Nodes that describe devices which generate interrupts must contain an
+"interrupts" property, an "interrupts-extended" property, or both. If both are
+present, the latter should take precedence; the former may be provided simply
+for compatibility with software that does not recognize the latter. These
+properties contain a list of interrupt specifiers, one per output interrupt. The
+format of the interrupt specifier is determined by the interrupt controller to
+which the interrupts are routed; see section 2 below for details.
 
   Example:
 	interrupt-parent = <&intc1>;