ath79: Add support for TP-Link CPE210 v3

ath79: Add support for TP-Link CPE210 v3

Specifications:

    * SoC: Qualcomm Atheros QCA9533 (650MHz)
    * RAM: 64MB
    * Storage: 8 MB SPI NOR
    * Wireless: 2.4GHz N based built into SoC 2x2
    * Ethernet: 1x 100/10 Mbps, integrated into SoC, 24V POE IN

Installation:
    Flash factory image through stock firmware WEB UI or TFTP
    To get to TFTP recovery just hold reset button while powering
    on for around 4-5 seconds and release.
    Rename factory image to recovery.bin
    Stock TFTP server IP:192.168.0.100
    Stock device TFTP adress:192.168.0.254

Thanks to robimarko for the work inside the ar71xx tree.
Thanks to adrianschmutzler for deep discussion and fixes.

Signed-off-by: Mario Schroen <m.schroen@web.de>
[Split into DTS/DTSI, read-only config partition in DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[renamed dtsi filename, light subject touches]
Signed-off-by: Petr Štetiar <ynezz@true.cz>