dt-bindings: bcm: Add Broadcom Stingray bindings document

This patch adds DT bindings info for Broadcom Stingray SOC
and related reference boards.
Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

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

+Broadcom Stingray device tree bindings
+------------------------------------------------
+
+Boards with Stingray shall have the following properties:
+
+Required root node property:
+
+Stingray Combo SVK board
+compatible = "brcm,bcm958742k", "brcm,stingray";
+
+Stingray SST100 board
+compatible = "brcm,bcm958742t", "brcm,stingray";