brcm47xx: add switch configuration for WNR3500L

brcm47xx: add switch configuration for WNR3500L

Netgear WNR3500L is an already supported device, but out of the
box, the device has no switch configuration and there is no wan.
The correct configuration for this specific model is similar to
some other models. This simple commit adds the correct switch
and the out-of-the-box experience is improved.

Experimentally determined:

Port 0 => WAN
Port 1..4 => LAN
Port 5..7 => unused
Port 8 => CPU

Signed-off-by: Olli Asikainen <olli.asikainen@gmail.com>
Tested-by: Fabian Zaremba <fabian@youremail.eu>
[added port mapping to commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>