net: dsa: Add support for platform data

Allow drivers to use the new DSA API with platform data. Most of the
code in net/dsa/dsa2.c does not rely so much on device_nodes and can get
the same information from platform_data instead.

We purposely do not support distributed configurations with platform
data, so drivers should be providing a pointer to a 'struct
dsa_chip_data' structure if they wish to communicate per-port layout.

Multiple CPUs port could potentially be supported and dsa_chip_data is
extended to receive up to one reference to an upstream network device
per port described by a dsa_chip_data structure.

dsa_dev_to_net_device() increments the network device's reference count,
so we intentionally call dev_put() to be consistent with the DT-enabled
path, until we have a generic notifier based solution.

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