ath79: add support for TP-Link TL-MR3420 v3

ath79: add support for TP-Link TL-MR3420 v3

This adds support for the TP-Link TL-MR3420 v3, a later revision of the
v2 with an external gpiochip similar to TP-Link Archer C7 v4.

Specifications:

SOC: Qualcomm Atheros QCA9531
CPU: 650MHz
Flash: 4 MiB
RAM: 32 MiB
WLAN: Qualcomm Atheros QCA9531 bgn 2T2R 2.4 GHz
Ethernet: 5 ports (100M)

Flashing instructions:

- Flash factory image from OEM WebUI:
  openwrt-ath79-tiny-tplink_tl-mr3420-v3-squashfs-factory.bin
- Sysupgrade from ath79 image:
  openwrt-ath79-tiny-tplink_tl-mr3420-v3-squashfs-sysupgrade.bin

Signed-off-by: Lim Guo Wei <limguowei@gmail.com>
[remove SUPPORTED devices, some typo adjustments, fix WAN MAC
address, fix sorting in 01_leds]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>