net: dsa: bcm_sf2: Make it a real platform device driver

The Broadcom Starfighter 2 switch driver should be a proper platform
driver, now that the DSA code has been updated to allow that, register a
switch device, feed it with the proper configuration data coming from
Device Tree and register our switch device with DSA.

The bulk of the changes consist in moving what bcm_sf2_sw_setup() did
into the platform driver probe function.

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