ath79: add support for D-Link DCH-G020 Rev. A1

ath79: add support for D-Link DCH-G020 Rev. A1

The DCH-G020 is a Smart Home Gateway for Z-Wave devices.

Specifications:

 * QCA9531, 16 MiB Flash, 64 MiB RAM
 * On-Board USB SD3503A Z-Wave dongle
 * GL850 USB 2.0 Hub (one rear port, internal Z-Wave)
 * Two Ethernet Ports (10/100)

Installation:

 * Web UI: http://192.168.0.60 (or different address obtained via DHCP)
   Login with 'admin' and the 6-digit PIN Code from the bottom label
 * Recovery Web UI: Keep reset button pressed during power-on
   until LED starts flashing red, upgrade via http://192.168.0.60
 * Some modern browsers may have problems flashing via the Web UI,
   if this occurs consider booting to recovery mode and flashing via:
   curl -F \
     files=@openwrt-ath79-generic-dlink_dch-g020-a1-squashfs-factory.bin \
     http://192.168.0.60/cgi/index

Known issues:

 * Real-Time-Clock is not working as there is currently no matching driver
   It is still included in the dts as compatible = "pericom,pt7c43390";
 * openzwave was tested on v19.07 (running MinOZW as a proof-of-concept),
   but the package grew too big as lots of device pictures were included,
   thus any use of Z-Wave is up to the user (e.g. extroot and domoticz)

The device will use the same MAC address for both wired and wireless
interfaces, however it is stored at two different locations in the flash.

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>