dt-bindings: net: dsa: ocelot: document the vsc9959 core

This patch adds the required documentation for the embedded L2 switch
inside the NXP LS1028A chip.

I've submitted it in the legacy format instead of yaml schema, because
DSA itself has not yet been converted to yaml, and this driver defines
no custom bindings.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>