Documentation: net: dsa: b53: Describe b53 configuration

Document the different needs of documentation for the b53 driver.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dsa/index.rst

@@ -6,6 +6,7 @@ Distributed Switch Architecture
    :maxdepth: 1
 
    dsa
+   b53
    bcm_sf2
    lan9303
    sja1105