dt-bindings: arm: bcm: Convert Stingray to YAML

Update the Broadcom Stingray SoC binding document for boards/SoCs to use
YAML. Verified with dt_binding_check and dtbs_check.

Acked-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>