ramips: add support for unbranded XDX-RN502J board

This patch adds support for routers using a PCB marked XDX-RN502J
V2.0, such as some unbranded routers like this:

http://www.mediafire.com/?8acv87h6snn9fj6
http://www.mediafire.com/?do7xonw7scor4kn
http://www.mediafire.com/?1ad49zvx3e7jyix
http://www.mediafire.com/?i46cqiq66maa197

http://www.aliexpress.com/product-fm/516397994-Free-Shipping-300M-3G-WAN-Wireless-N-WiFi-USB-AP-Router-2-Antennas-wholesalers.html

some reports also state the Aceex 22Y uses the same board.

more information can be found in the following openwrt forum post
https://forum.openwrt.org/viewtopic.php?id=34878

[juhosg: fix checkpatch warnings, use the GENERIC_4M template]

Signed-off-by: bruno schwander <bruno@tinkerbox.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@30809 3c298f89-4303-0410-b956-a3cf2f4a3e73