ipq60xx: add support for ALFA Network AP120C-AX

ipq60xx: add support for ALFA Network AP120C-AX

ALFA Network AP120C-AX is a dual-band ceiling AP, based on Qualcomm
IPQ6000 + QCN5021 + QCN5052 + QCA8072 chipsets bundle.

Specifications:

- SOC:      Qualcomm IPQ6000 (quad-core Cortex-A53 1.2 GHz)
- DRAM:     DDR3 512 MB (Micron MT41K256M16TW-107)
- Flash:    16 MB SPI NOR (Macronix MX25U12832F, boot device)
            128 MB NAND (Macronix MX30UF1G18AC, dual-firmware)
- Ethernet: 2x 10/100/1000 Mbps Ethernet (QCA8072)
            802.3at/af PoE input in WAN port
- Wi-Fi:    2x2 2.4 GHz Wi-Fi 6 (QCN5021 + RFFM8227 FEM)
            2x2   5 GHz Wi-Fi 6 (QCN5152 + QPF4568 FEM)
- Antenna:  for indoor version: dual-band, internal
            2x (or 4x) U.FL antenna connectors on the PCB
- LED:      for indoor/outdoor versions: 5x on external module (status,
            2x Wi-Fi, 2x Ethernet), PoE LED on-board
            8-pin on-board header for LED module (1.27 mm pitch, J14)
- Button:   1x button (reset)
- USB:      1x 4-pin on-board header for USB 2.0 (2.54 mm pitch, J22)
- UART:     1x micro USB Type-B for system console (Holtek HT42B534)
            1x 4-pin on-board header (2.54 mm pitch, J11)
- Power:    802.3at/af PoE or 12 V DC/2 A (DC jack)
- Other:    8-pin and 4-pin on-board headers for external Bluetooth
            module (1.27 mm pitch, J15, J16, unavailable, thus untested)

MAC addresses:

- WAN:               00:c0:ca:xx:xx:6c (art 0x0, device's label -2)
- LAN:               00:c0:ca:xx:xx:6d (art 0x6, device's label -1)
- 2.4 GHz (IPQ6000): 00:c0:ca:xx:xx:6e (art 0xc, device's label)
-   5 GHz (IPQ6000): 00:c0:ca:xx:xx:6f (device's label + 1)

Flash instructions:

Due to the lack of direct GUI based update capability and dual-firmware
partition configuration, it is recommended to use TFTP + serial console
based approach (console is available in micro USB connector):

1. Set a static IP 192.168.1.1/24 on PC and start TFTP server with the
   '...-factory.ubi' image renamed to 'firmware.bin'.
2. Make sure you can access board's serial console over micro USB.
3. Power up the device, hit any key to enter U-Boot CLI and issue below
   commands.

3.1 Restore U-Boot's environment to default values (double check first
    the '0:APPSBLENV' partition offset using 'smem' command):

    sf probe
    sf erase 0x510000 0x10000
    saveenv

3.2 Download and install OpenWrt in both partitions and reset the board:

    tftpb 0x44000000 firmware.bin
    flash rootfs
    flash rootfs_1
    reset

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>