ath79: add support for TP-Link Archer C7 v1

ath79: add support for TP-Link Archer C7 v1

TP-Link Archer C7 v1 is a dual band router
based on Qualcomm/Atheros QCA9558 + QCA9880.

Specification:

 - 720 MHz CPU
 - 128 MB of RAM (Various chips)
 - 8 MB of FLASH (Various chips)
 - SoC QCA9558 integrated 3T3R 2.4 GHz Wi-Fi
 - minipcie slot with 3T3R 5 GHz QCA9880-AR1A (unsupported by ath10k!)
 - 5x 10/100/1000 Mbps Ethernet (AR8327N Switch)
 - 10x LEDs, 2x software buttons

For further informwation on the device, visit the wiki:
<https://openwrt.org/toh/tp-link/archer-c7-1750>

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>