dt-bindings: bus: Document breakpoint interrupt for gisb-arb

The GISB arbiter can have a third and optional interrupt to handle GISB
breakpoints.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/bus/brcm,gisb-arb.txt

@@ -10,7 +10,8 @@ Required properties:
     "brcm,bcm7038-gisb-arb" for 130nm chips
 - reg: specifies the base physical address and size of the registers
 - interrupts: specifies the two interrupts (timeout and TEA) to be used from
-  the parent interrupt controller
+  the parent interrupt controller. A third optional interrupt may be specified
+  for breakpoints.
 
 Optional properties: