ARM: dts: kirkwood-linksys-viper: Utilize new DSA binding

Utilize the new DSA binding, introduced with commit 8c5ad1d6

 ("net: dsa:
Document new binding"). The legacy binding node is kept included, but is marked
disabled.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>