MAINTAINERS: B53: Add DT binding file

When the binding was added with 967dd82ffc ("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>

MAINTAINERS

@@ -3388,6 +3388,7 @@ M:	Florian Fainelli <f.fainelli@gmail.com>
 L:	netdev@vger.kernel.org
 L:	openwrt-devel@lists.openwrt.org (subscribers-only)
 S:	Supported
+F:	Documentation/devicetree/bindings/net/dsa/b53.txt
 F:	drivers/net/dsa/b53/*
 F:	include/linux/platform_data/b53.h