Documentation: bindings: brcmstb: Document write-pairing

Document the hif-cpubiuctrl node a bit more, and add a documentation
entry for the optional "brcm,write-pairing" property.

Acked-by: Rob Herring <[e-mail]>
Acked-by: Gregory Fong <[e-mail]>
Signed-off-by: Florian Fainelli <[e-mail]>

Documentation/devicetree/bindings/arm/bcm/brcm,brcmstb.txt

···
     - compatible: "brcm,bcm<chip_id>-hif-cpubiuctrl", "syscon"
     - compatible: "brcm,bcm<chip_id>-hif-continuation", "syscon"
 
+hif-cpubiuctrl node
+-------------------
+SoCs with Broadcom Brahma15 ARM-based CPUs have a specific Bus Interface Unit
+(BIU) block which controls and interfaces the CPU complex to the different
+Memory Controller Ports (MCP), one per memory controller (MEMC). This BIU block
+offers a feature called Write Pairing which consists in collapsing two adjacent
+cache lines into a single (bursted) write transaction towards the memory
+controller (MEMC) to maximize write bandwidth.
+
+Required properties:
+
+    - compatible: must be "brcm,bcm7445-hif-cpubiuctrl", "syscon"
+
+Optional properties:
+
+    - brcm,write-pairing:
+	Boolean property, which when present indicates that the chip
+	supports write-pairing.
+
 example:
     rdb {
         #address-cells = <1>;
···
         hif_cpubiuctrl: syscon@3e2400 {
             compatible = "brcm,bcm7445-hif-cpubiuctrl", "syscon";
             reg = <0x3e2400 0x5b4>;
+            brcm,write-pairing;
         };
 
         hif_continuation: syscon@452000 {