ath79: add support for TP-Link TL-WR710N v2.1

ath79: add support for TP-Link TL-WR710N v2.1

This adds support for the TP-Link TL-WR710N v2.1. It is basically a
re-issue of the v1.2.

Specifications:

SoC:       Atheros AR9331
CPU:       400 MHz
Flash:     8 MiB
RAM:       32 MiB
WiFi:      2.4 GHz b/g/n
Ethernet:  2x 100M ports
USB:       1x 2.0

The only difference from the v1 is the TP-Link hardware ID/revision.

Attention:
The TL-WR710N v2.0 (!) has only 4 MB flash and cannot be flashed with
this image. It has a different TPLINK_HWREV, so accidental flashing
of the factory image should be impossible without additional measures.

Unfortunately, the v2.0 in ar71xx has the same board name, so sysupgrade
from ar71xx v2.0 into ath79 v1/v2.1 will not be prevented, but will brick
the device.

Flashing instruction:

Upload the factory image via the OEM firmware GUI upgrade mechanism.

Further notes:

To make implementation easier if somebody desires to port the 4M v2.0,
this already creates two DTSI files.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Fabian Eppig <fabian@eppig.de>
(backported from eb531337a779a48a2d17bc66f0d222325d6c1563)