ar71xx: add support for TP-LINK Archer C7 v5

ar71xx: add support for TP-LINK Archer C7 v5

TP-Link Archer C7 v5 is a dual-band AC1750 router, based on Qualcomm/Atheros
QCA9563+QCA9880.

Specification:

- 750/400/250 MHz (CPU/DDR/AHB
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 3T3R 5 GHz
- 5x 10/100/1000 Mbps Ethernet
- 10x LED, 2x button
- UART header on PCB

Flash instruction:
1. Upload lede-ar71xx-generic-archer-c7-v5-squashfs-factory.bin via Web interface

Flash instruction using TFTP recovery:
1. Set PC to fixed ip address 192.168.0.66
2. Download lede-ar71xx-generic-archer-c7-v5-squashfs-factory.bin
and rename it to ArcherC7v5_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: Arvid E. Picciani <aep@exys.org>
(cherry picked from commit bf39d5594b3c8f9409e6d9408a1f370c9f18d0dd)