dt-bindings: arm: bcm: Convert Northstar 2 to YAML

Update the Broadcom Northstar 2 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>