net: dsa: b53: Provide sensible defaults

The SRAB driver is the default way to communicate with the integrated
switch on iProc platforms and the MMAP driver is the way to communicate
with the integrated switch on DSL BCM63xx and CM BCM33xx.

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