ath79: add support for D-link DAP-1720 A1

ath79: add support for D-link DAP-1720 A1

D-Link DAP-1720 rev A1 is a mains-powered AC1750 Wi-Fi range extender,
manufactured by Alpha Networks [8WAPAC28.1A1G].
(in square brackets: PCB silkscreen markings)

Specifications:
* CPU (Qualcomm Atheros QCA9563-AL3A [U5]):
  775 MHz single core MIPS 74Kc;
* RAM (Winbond W9751G6KB-25J [U3]):
  64 MiB DDR2;
* ROM (Winbond W25Q128FV [U16]):
  16 MiB SPI NOR flash;
* Ethernet (AR8033-AL1A PHY [U1], no switch):
  1 GbE RJ45 port (no PHY LEDs);
* Wi-Fi
  * 2.4 GHz (Qualcomm Atheros QCA9563-AL3A [U5]):
    3x3 802.11n;
  * 5 GHz (Qualcomm Atheros QCA9880-BR4A [U9]):
    3x3 802.11ac Wave 1;
  * 3 foldable dual-band antennas (U.fl) [P1],[P2],[P3];
* GPIO LEDs:
  * RSSI low (red/green) [D2];
  * RSSI medium (green) [D3];
  * RSSI high (green) [D4];
  * status (red/green) [D5];
* GPIO buttons:
  * WPS [SW1], co-located with status LED;
  * reset [SW4], accessible via hole in the side;
* Serial/UART:
  Tx-Gnd-3v3-Rx [JP1], Tx is the square pin, 1.25mm pitch;
  125000-8-n-1 in U-boot, 115200-8-n-1 in kernel;
* Misc:
  * 12V VCC [JP2], fed from internal 12V/1A AC to DC converter;
  * on/off slide switch [SW2] (disconnects VCC mechanically);
  * unpopulated footprints for a Wi-Fi LED [D1];
  * unpopulated footprints for a 4-pin 3-position slide switch (SW3);

MAC addresses:
* Label = LAN;
* 2.4 GHz WiFi = LAN;
* 5 GHz WiFi = LAN+2;

Installation:
* `factory.bin` can be used to install OpenWrt from OEM firmware via the
  standard upgrade webpage at http://192.168.0.50/UpdateFirmware.html
* `recovery.bin` can be used to install OpenWrt (or revert to OEM
  firmware) from D-Link Web Recovery. To enter web recovery, keep reset
  button pressed and then power on the device. Reset button can be
  released when the red status LED is bright; it will then blink slowly.
  Set static IP to 192.168.0.10, navigate to http://192.168.0.50 and
  upload 'recovery.bin'. Note that in web recovery mode the device
  ignores ping and DHCP requests.

Note: 802.11s is not supported by the default `ath10k` driver and
firmware, but is supported by the non-CT driver and firmware variants.
The `-smallbuffers` driver variant is recommended due to RAM size.

Co-developed-by: Anthony Sepa <protectivedad@gmail.com>
Signed-off-by: Rani Hod <rani.hod@gmail.com>