ath79: add support for Netgear WNDR4500 v3
This patch introduces support for Netgear WNDR4500v3. Router
is very similar to WNDR4300v2 and is based on the same PCB.
Information gathered from various Internet sources (including
https://patchwork.ozlabs.org/patch/809227/) shows following
differences to WNDR4300v2:
* two USB 2.0 ports with separate LEDs
* USB LEDs soldered to secondary pads
* WPS and RFKILL buttons soldered to secondary pads
* described as N900 device with 3x3:3 MIMO for 2.4GHz radio
* power supply requirement is DC 12V 2.5A
* vendor HW ID suffix differs in one digit
* bigger chassis
Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>