Doc: net: dsa: b53: update location of referenced dsa.txt

The referenced file dsa.txt is located at
Documentation/devicetree/bindings/net/dsa/dsa.txt

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>