ar71xx: Add TP-Link TL-WR740N v6 support to trunk

This patch adds support for the TP-Link TL-WR740N v6 into OpenWrt trunk. Hardware wise, it's very similar to the TL-WR841N v10. See the forum thread [1]. Credit goes to the user 'traveler' who did the legwork and tested builds.

Also thanks to Matthias Schiffer for clarifying the DEVICE_PROFILE settings.

Signed-off by: Stijn Segers <francesco.borromini@inventati.org>

[1]: https://forum.openwrt.org/viewtopic.php?pid=318704

SVN-Revision: 49245