ar71xx: add support for TP-Link Archer C60 v2

ar71xx: add support for TP-Link Archer C60 v2

TP-Link Archer C60 v2 is a dual-band AC1350 router, based on
Qualcomm/Atheros QCA9561 + QCA9886.

Specification:

- 775/650/258 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 2T2R 5 GHz
- 5x 10/100 Mbps Ethernet
- 7x LED, 2x button
- UART header on PCB

Flash instruction (web):

Download lede-ar71xx-generic-archer-c60-v2-squashfs-factory.bin and use
OEM System Tools - Firmware Upgrade site.

Flash instruction (recovery):

1. Set PC to fixed IP address 192.168.0.66
2. Download lede-ar71xx-generic-archer-c60-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

Flash instruction (under U-Boot, using UART):

tftp 0x81000000 lede-ar71xx-...-sysupgrade.bin
erase 0x9f030000 +$filesize
cp.b $fileaddr 0x9f030000 $filesize
reset

Signed-off-by: Henryk Heisig <hyniu@o2.pl>