board: gw_ventana: enable MV88E61XX DSA support

board: gw_ventana: enable MV88E61XX DSA support

Add MV88E61XX DSA support:
 - update dt to provide internal MDIO bus and port handles.
   U-Boot requires a more restrictive subset of the dt bindings
   required by Linux for the sake of simplifying code
 - update defconfig to remove old driver and enable new one
 - replace mv88e61xx_hw_reset weak override with board_phy_config support
   for register configuration that is outside the scope of the DSA driver

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>