ar71xx: add TP-LINK TL-WR841N/ND v12 image

This router has the same hardware as TP-LINK TL-WR841N/ND v11 (same FCC
ID, same TFTP image name...).

If the stock firmware web interface doesn't accept LEDE factory image,
it can be flashed via the U-Boot TFTP recovery mode, by long-pressing
the reset button after power on.

The TFTP image name is wr841nv11_tp_recovery.bin (yes, v11, not v12).

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>