ath79: add support for D-Link DIR-859 A3

ath79: add support for D-Link DIR-859 A3

Specifications:
  SOC:      QCA9563 775 MHz + QCA9880
  Switch:   QCA8337N-AL3C
  RAM:      Winbond W9751G6KB-25 64 MiB
  Flash:    Winbond W25Q128FVSG 16 MiB
  WLAN:     Wi-Fi4 2.4 GHz 3*3 + 5 GHz 3*3
  LAN:      LAN ports *4
  WAN:      WAN port *1
  Buttons:  reset *1 + wps *1
  LEDs: ethernet *5, power, wlan, wps

MAC Address:
  use      address               source1          source2
  label    40:9b:xx:xx:xx:3c     lan && wlan      u-boot,env@ethaddr
  lan      40:9b:xx:xx:xx:3c     devdata@0x3f     $label
  wan      40:9b:xx:xx:xx:3f     devdata@0x8f     $label + 3
  wlan2g   40:9b:xx:xx:xx:3c     devdata@0x5b     $label
  wlan5g   40:9b:xx:xx:xx:3e     devdata@0x76     $label + 2

Install via Web UI:
  Apply factory image in the stock firmware's Web UI.

Install via Emergency Room Mode:
  DIR-859 A1 will enter recovery mode when the system fails to boot
  or press reset button for about 10 seconds.

  First, set computer IP to 192.168.0.5 and Gateway to 192.168.0.1.
  Then we can open http://192.168.0.1 in the web browser to upload
  OpenWrt factory image or stock firmware. Some modern browsers may
  need to turn on compatibility mode.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>