net: dsa: mv88e6xxx: rework FDB getnext operation

This commit adds a low level _mv88e6xxx_atu_getnext function and helpers
to rewrite the mv88e6xxx_port_fdb_getnext operation.

A mv88e6xxx_atu_entry structure is added for convenient access to the
hardware, and GLOBAL_ATU_FID is defined instead of the raw 0x01 value.

The previous implementation did not handle the eventual trunk mapping.
If the related bit is set, then the ATU data register would contain the
trunk ID, and not the port vector.

Check this in the FDB getnext operation and do not handle it (yet).

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