dt-bindings: net: dsa: qca8k: document support for qca8328

QCA8328 is the bigger brother of qca8327. Document the new compatible
binding and add some information to understand the various switch
compatible.

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