net: dsa: Split platform data to header file

Instead of having net/dsa.h contain both the internal switch tree/driver
structures, split the relevant platform_data parts into
include/linux/platform_data/dsa.h and make that header be included by
net/dsa.h in order not to break any setup. A subsequent set of patches
will update code including net/dsa.h to include only the platform_data
header.

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