ath79: add support for TP-Link Archer C7 v5

ath79: add support for TP-Link Archer C7 v5

This commit adds support for TP-Link Archer C7 v5, leveraging most effort
from commit ea9baee and 1e4ee63. Archer C7 v5 is identical to Archer A7 v5
but with a different flash layout.

Specification:
-   QCA9563 SoC (750 MHz)
-   128 MiB of RAM (DDR2)
-   16 MiB of flash (SPI)
-   5x 1 Gbps Ethernet (1x WAN + 4x LAN)
-   2.4GHz (bgn) SoC internal + 5GHz (ac) QCA9880
-   10x LED, 2x button
-   UART header on PCB

Flash instructions:
1.  Upload openwrt-ath79-generic-tplink_archer-c7-v5-squashfs-factory.bin
    via web interface.

Flash instructions using TFTP recovery:
1.  Plug PC to one of the LAN ports
2.  Set PC to fixed IP address 192.168.0.66
3.  Rename the factory image to ArcherC7v5_tp_recovery.bin and place it in
    TFTP root directory
4.  Turn on the router with the reset button pressed for about 15 secs
5.  Release the button and wait about 150 secs to complete flashing

Signed-off-by: TOCK Chiu <tock.chiu@gmail.com>