net: dsa: b53: Allow SRAB driver to specify platform data

For Northstart Plus SoCs, we cannot detect the switch because only the
revision information is provied in the Management page, instead, rely on
Device Tree to tell us the chip id, and pass it down using the
b53_platform_data structure.

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