dt-bindings: net: dsa: add bindings for xrs700x switches

Add documentation and an example for Arrow SpeedChips XRS7000 Series
single chip Ethernet switches.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>