net: dsa: b53: Prepare to support 7445 switch

Allocate a device entry for the Broadcom BCM7445 integrated switch
currently backed by bcm_sf2.c. Since this is the latest generation, it
has 4 ARL entries, 4K VLANs and uses Port 8 for the CPU/IMP port.

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