net: dsa: mv88e6xxx: Add devlink regions

Allow the global registers, and the ATU to be snapshot via devlink
regions. It is later planned to add support for the port registers.

v2:
Remove left over debug prints
Comment ATU format is generic for mv88e6xxx, not wider

v3:
Make use of ops structure passed to snapshot function
Remove port regions

v4:
Make use of enum mv88e6xxx_region_id
Fix global2/global1 read typ0

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>