Documentation: networking: dsa: Update documentation

Since 83c0afae

 ("net: dsa: Add new binding implementation"), DSA is
no longer a platform device exclusively and can support registering DSA
switches from other bus drivers (PCI, USB, I2C, etc.).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>