ath79: add support for TP-Link TL-WR841N/ND v12

ath79: add support for TP-Link TL-WR841N/ND v12

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

Flash instruction (WebUI):
Download *-factory.bin image and upload it via the firmwary upgrade
function of the stock firmware WebUI.

Flash instruction (TFTP):
1. Set PC to fixed ip address 192.168.0.66
2. Download *-factory.bin image and rename it to wr841nv11_tp_recovery.bin
   (it's really v11, not v12)
3. Start a tftp server with the image file in its root directory
4. Turn off the router
5. Press and hold Reset button
6. Turn on router with the reset button pressed and wait ~15 seconds
7. Release the reset button and after a short time
the firmware should be transferred from the tftp server
8. Wait ~30 second to complete recovery.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 8b76c6695b9274e3c12fdf80bcad7dc0c7202585)