arm64: dts: rename ns2.txt to brcm,ns2.txt

Rename ns2.txt to brcm,ns2.txt to match naming convention followed
by rest of Broadcom binding documentation.
Signed-off-by: NScott Branden <scott.branden@broadcom.com>
Reviewed-by: NRay Jui <ray.jui@broadcom.com>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>