ar71xx: add support for TP-Link Archer C59 v2

ar71xx: add support for TP-Link Archer C59 v2

TP-Link Archer C59v2 is a dual-band AC1350 router based on
Qualcomm/Atheros QCA9561+QCA9886 chips.

Specification:

- 775/650/258 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 2T2R 5 GHz
- 5x 10/100 Mbps Ethernet
- USB 2.0 port
- UART header on PCB

Flash instruction:
  - via web UI:
    1. Download openwrt-ar71xx-generic-archer-c59-v2-squashfs-factory.bin
    2. Login to router and open the Advanced tab
    3. Navigate to System Tools -> Firmware Upgrade
    4. Upload firmware using the Manual Upgrade form
  - via TFTP:
    1. Set PC to fixed ip address 192.168.0.66
    2. Download openwrt-ar71xx-generic-archer-c59-v2-squashfs-factory.bin
       and rename it to tp_recovery.bin
    3. Start a tftp server with the file tp_recovery.bin in its root directory
    4. Turn off the router
    5. Press and hold Reset button
    6. Turn on router with the reset button pressed and wait ~15 seconds
    7. Release the reset button and after a short time
       the firmware should be transferred from the tftp server
    8. Wait ~30 second to complete recovery.

Signed-off-by: Keith Maika <keithm@aoeex.com>