net: dsa: bcm_sf2: setup BCM4908 internal crossbar

On some SoCs (e.g. BCM4908, BCM631[345]8) SF2 has an integrated
crossbar. It allows connecting its selected external ports to internal
ports. It's used by vendors to handle custom Ethernet setups.

BCM4908 has following 3x2 crossbar. On Asus GT-AC5300 rgmii is used for
connecting external BCM53134S switch. GPHY4 is usually used for WAN
port. More fancy devices use SerDes for 2.5 Gbps Ethernet.

              ┌──────────┐
SerDes ─── 0 ─┤          │
              │   3x2    ├─ 0 ─── switch port 7
 GPHY4 ─── 1 ─┤          │
              │ crossbar ├─ 1 ─── runner (accelerator)
 rgmii ─── 2 ─┤          │
              └──────────┘

Use setup data based on DT info to configure BCM4908's switch port 7.
Right now only GPHY and rgmii variants are supported. Handling SerDes
can be implemented later.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>