net: dsa: add Broadcom SF2 switch driver

Add support for the Broadcom Starfigther 2 switch chip using a DSA
driver. This switch driver supports the following features:

- configuration of the external switch port interface: MII, RevMII,
  RGMII and RGMII_NO_ID are supported
- support for the per-port MIB counters
- support for link interrupts for special ports (e.g: MoCA)
- powering up/down of switch memories to conserve power when ports are
  unused

Finally, update the compatible property for the DSA core code to match
our switch top-level compatible node.

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