ath79: add support for the TP-LINK CPE220 V3

ath79: add support for the TP-LINK CPE220 V3

This adds support for a popular low-cost 2.4GHz N based AP

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

Installation:
Flash factory image through stock firmware WEB UI
or through 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

This also applies some minor changes to the common DTSI:
- use &wmac for label-mac-device, as this one is actually set up in
  common DTSI
- move &eth0 to parent DTSI
- fix several leading spaces, added/removed newlines

Signed-off-by: Andrew Cameron <apcameron@softhome.net>
[DTS style fixes/improvements, updated commit message/title,
backport to 19.07]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>