MAINTAINERS: B53: Add DT binding file

When the binding was added with 967dd82f ("net: dsa: b53: Add
support for Broadcom RoboSwitch"), it was not explicitly added to the
B53 MAINTAINERS file section, add it now.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>