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>