ath79: add support for D-Link COVR-P2500 A1

ath79: add support for D-Link COVR-P2500 A1

Specifications:
* QCA9563, 16 MiB flash, 128 MiB RAM, 2T2R 802.11n
* QCA9886 2T2R 801.11ac Wave 2
* QCA7550 Homeplug AV2 1300
* AR8337, 3 Gigabit ports (1, 2: LAN; 3: WAN)

To make use of PLC functionality, firmware needs to be
provided via plchost (QCA7550 comes without SPI NOR),
patched with the Network Password and MAC.

Flashing via OEM Web Interface
* Flash 'factory.bin' using web-interface
* Wait until firmware succesfully installed and device booted
* Hold down reset button to reset factory defaults (~10 seconds)

Flashing via Recovery Web Interface:
* Hold down reset button during power-on (~10 seconds)
* Recovery Web UI is at 192.168.0.50, no DHCP.
* Flash 'recovery.bin' with
  scripts/flashing/dlink_recovery_upload.py
  (Recovery Web UI does not work with modern OSes)

Return to stock
* Hold down reset button during power-on (~10 seconds)
* Recovery Web UI is at 192.168.0.50, no DHCP.
* Flash unencrypted stock firmware with
  scripts/flashing/dlink_recovery_upload.py
  (Recovery Web UI does not work with modern OSes)

Co-developed-by: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Daniel Linjama <daniel@dev.linjama.com>