ar71xx: Add support for PISEN TS-D084

ar71xx: Add support for PISEN TS-D084

PISEN TS-D084 is an wireless router with a battery and integrated power supply based on Atheros AR9331.

Specification:

- 400/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH (SPI NOR)
- 1x 10/100 Mbps Ethernet
- 1T1R 2.4 GHz (AR9331)
- 1x USB 2.0

Flash instruction:
The manufacturer are using exactly the same firmware header as TP-LINK TL-WR703N (including device ID!). Simply upload the factory firmware into WebUI and flashing is done.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
(cherry picked from commit a789c0f49159c618d990541c23e00630f7296ebc)