net: dsa: mv88e6xxx: Provide function for common port initialization

Provide mv88e6xxx_setup_port_common() for common port initialization.
Currently only write Port 1 Control and VLAN configuration since
this will be needed for hardware bridging. More can be added later
if desired/needed.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>