MAINTAINERS: Add file patterns for dsa device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -9882,6 +9882,7 @@ M:	Andrew Lunn <andrew@lunn.ch>
 M:	Vivien Didelot <vivien.didelot@savoirfairelinux.com>
 M:	Florian Fainelli <f.fainelli@gmail.com>
 S:	Maintained
+F:	Documentation/devicetree/bindings/net/dsa/
 F:	net/dsa/
 F:	include/net/dsa.h
 F:	include/linux/dsa/