ath79: add support for D-Link COVR-C1200 A1

ath79: add support for D-Link COVR-C1200 A1

The COVR-C1200 devices are sold as "Whole Home Mesh Wi-Fi"
sets in packs of two (COVR-C1202) and three (COVR-C1203).

Specifications:
 * QCA9563, 16 MiB flash, 128 MiB RAM, 2x3:2 802.11n
 * QCA9886 2x2:2 801.11ac Wave 2
 * AR8337, 2 Gigabit ports (1: WAN; 2: LAN)
 * USB Type-C power connector (5V, 3A)

Installation COVR Point A:
 * In factory reset state: OEM Web UI is at 192.168.0.50
   no DHCP, skip wizard by directly accessing:
     http://192.168.0.50/UpdateFirmware_Simple.html
 * After completing setup wizard: Web UI is at 192.168.0.1
     DHCP enabled, login with empty password
 * Flash factory.bin
 * Perform a factory reset to restore OpenWrt UCI defaults

Installation COVR Points B:
 * OEM Web UI is at 192.168.0.50, no DHCP, empty password
 * Flash factory.bin
 * Perform a factory reset to restore OpenWrt UCI defaults

Recovery:
 * Keep reset button pressed during power on
 * Recovery Web UI is at 192.168.0.50, no DHCP
 * Flash factory.bin
   used to work best with Chromium-based browsers or curl:
     curl -F firmware=@factory.bin \
       http://192.168.0.50/upgrade.cgi
   since this fails to work on modern Linux systems,
   there is also a script dlink_recovery_upload.py

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