dt-bindings: interrupt-controller: Document UPG auxiliary L2

Define the compatible string brcm,upg-aux-aon-l2-intc which is used by
the Broadcom STB UPG auxiliary always-on interrupt controller.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200709223016.989-6-f.fainelli@gmail.com

Documentation/devicetree/bindings/interrupt-controller/brcm,l2-intc.txt

@@ -4,6 +4,7 @@ Required properties:
 
 - compatible: should be one of:
 	      "brcm,hif-spi-l2-intc" or
+	      "brcm,upg-aux-aon-l2-intc" or
 	      "brcm,l2-intc" for latched interrupt controllers
               should be "brcm,bcm7271-l2-intc" for level interrupt controllers
 - reg: specifies the base physical address and size of the registers