net: dsa: Include platform_data header file

b53 and mv88e6xxx support passing platform_data, and now that we have
split the platform_data portion from the main net/dsa.h header file,
include only the relevant parts.

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