dt-bindings: Create Documentation for NSP DT bindings

Add the documentation for the Broadcom Northstar Plus device tree
bindings.
Signed-off-by: Jon Mason <jonmason@broadcom.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

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

+Broadcom Northstar Plus device tree bindings
+--------------------------------------------
+
+Broadcom Northstar Plus family of SoCs are used for switching control
+and management applications as well as residential router/gateway
+applications. The SoC features dual core Cortex A9 ARM CPUs, integrating
+several peripheral interfaces including multiple Gigabit Ethernet PHYs,
+DDR3 memory, PCIE Gen-2, USB 2.0 and USB 3.0, serial and NAND flash,
+SATA and several other IO controllers.
+
+Boards with Northstar Plus SoCs shall have the following properties:
+
+Required root node property:
+
+BCM58522
+compatible = "brcm,bcm58522", "brcm,nsp";
+
+BCM58525
+compatible = "brcm,bcm58525", "brcm,nsp";
+
+BCM58535
+compatible = "brcm,bcm58535", "brcm,nsp";
+
+BCM58622
+compatible = "brcm,bcm58622", "brcm,nsp";
+
+BCM58623
+compatible = "brcm,bcm58623", "brcm,nsp";
+
+BCM58625
+compatible = "brcm,bcm58625", "brcm,nsp";
+
+BCM88312
+compatible = "brcm,bcm88312", "brcm,nsp";