dt-bindings: interrupt-controller: Document Broadcom STB HIF L2

Add documentation for the brcm,hif-spi-l2-intc compatible string to the
brcm,l2-intc.txt binding document.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200709223016.989-4-f.fainelli@gmail.com

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

@@ -2,7 +2,9 @@ Broadcom Generic Level 2 Interrupt Controller
 
 Required properties:
 
-- compatible: should be "brcm,l2-intc" for latched interrupt controllers
+- compatible: should be one of:
+	      "brcm,hif-spi-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
 - interrupt-controller: identifies the node as an interrupt controller