ath79: add support for TP-LINK Archer C7 v4

ath79: add support for TP-LINK Archer C7 v4

TP-Link Archer C7 v4 is a dual-band AC1750 router, based on the
Qualcomm/Atheros QCA9561 SoC + QCA9880.

Specification:

- 775/650/258 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
- 7x LED, 2x button
- UART header on PCB

Flash instruction:
1. Upload openwrt-ath79-generic-tplink_archer-c7-v4-squashfs-factory.bin
   via Web interface

Flash instruction using TFTP recovery:
1. Set PC to fixed ip address 192.168.0.66
2. Download openwrt-ath79-generic-tplink_archer-c7-v4-squashfs-factory.bin
   and rename it to ArcherC7v4_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: Oldřich Jedlička <oldium.pro@gmail.com>