net: dsa: qca8k: Add QCA8334 binding documentation

Add support for the four-port variant of the Qualcomm QCA833x switch.

The CPU port default link settings can be reconfigured using
a fixed-link sub-node.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>