net: dsa: mt7530: Add support for port 5

Adding support for port 5.

Port 5 can muxed/interface to:
- internal 5th GMAC of the switch; can be used as 2nd CPU port or as
  extra port with an external phy for a 6th ethernet port.
- internal PHY of port 0 or 4; Used in most applications so that port 0
  or 4 is the WAN port and interfaces with the 2nd GMAC of the SOC.

Signed-off-by: René van Dorst <opensource@vdorst.com>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>