ath79: add support for TP-Link TL-WDR7500 v3

ath79: add support for TP-Link TL-WDR7500 v3

This ports support for the TP-Link TL-WDR7500 v3 from ar71xx to ath79.

The basic features appear to be identical to the Archer C7 v1, however
it has the (supported) QCA9880-BR4A chip of the C7 v2.

Specifications:

  SoC:       QCA9558
  CPU:       720 MHz
  Flash:     8 MiB
  RAM:       128 MiB
  WLAN:      2.4 GHz b/g/n, 5 GHz a/n/ac
             Qualcomm Atheros QCA9880-BR4A
  Ethernet:  5x Gbit ports
  USB:       2x 2.0 ports

Flashing instructions:

Upload the factory image via the OEM firmware GUI.

TFTP recovery appears to be available as well.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>