arm64: dts: Add DT node to enable BGMAC driver on Stingray

This patch adds DT node to enable BGMAC driver on Stingray

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Oza Oza <oza.oza@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>