ar71xx: add support for Anonabox Pro

Chipset is a 650mhz Atheros with 128mb ram.
Variation of QCA9531 SoC with two Ethernet ports
and a 802.11n 2.4 GHz radio.

Signed-off-by: August Germar <august@anonabox.com>
[with review by Daniel Engberg]
Reviewed-by: Zoltan HERPAI <wigyori@uid0.hu>