ath79: add support for TP-Link TL-WA830RE v1

ath79: add support for TP-Link TL-WA830RE v1

This ports support for the TL-WA830RE v1 range extender from ar71xx to
ath79.

Specifications:
- SOC: Atheros AR7240
- CPU: 400MHz
- Flash: 4 MiB (Spansion S25FL032P)
- RAM: 32 MiB (Zentel A3S56D40FTP-G5)
- WLAN: Atheros AR9280 bgn 2x2
- Ethernet: 1 port (100M)

Flash instructions:
- install from u-boot with tftp (requires serial access)
  > setenv ipaddr a.b.c.d
  > setenv serverip e.f.g.h
  > tftpboot 0x80000000 \
      openwrt-ath79-tiny-tplink_tl-wa830re-v1-squashfs-factory.bin
  > erase 0x9f020000 +0x3c0000
  > cp.b 0x80000000 0x9f020000 0x3c0000
  > bootm 0x9f020000
- flash factory image from OEM WebUI
- sysupgrade from ar71xx image

The device seems to be a clone of the following devices not yet
added to ath79:
- tl-wa701nd-v1
- tl-wa730re-v1
- tl-wa801nd-v1
- tl-wa901nd-v1

Signed-off-by: Christian Buschau <christian.buschau@mailbox.org>
[make use of ar7240_tplink.dtsi, add note about clones]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit fb99ac6807f29eb5fe50271eff711e7415038731)