net: dsa: mv88e6xxx: add port files

The Marvell switches contains one internal SMI device per port, called
"Port Registers". Depending on the model, the addresses of these devices
start from 0x0, 0x8 or 0x10.

Start moving Port Registers specific code to their own files.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>