Documentation: networking: dsa: Remove platform device TODO

Since commit 83c0afae ("net: dsa: Add new binding implementation"),
the shortcomings of the dsa platform device have been addressed, remove
that TODO item.

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