net: dsa: b53: Define SF2 MIB layout

The 58xx and 7445 chips use the Starfighter2 code, define its MIB layout
and introduce a helper function: is58xx() which checks for both of these
IDs for now.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>