ramips: add support for TP-Link TL-MR3420 v5

ramips: add support for TP-Link TL-MR3420 v5

TP-Link TL-MR3420 v5 are simple N300 router with
5-port FE switch and non-detachable antennas.
Its very similar to TP-Link TL-WR841N V13.

Specification:

- MT7628N/N (580 MHz)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH
- 2T2R 2.4 GHz
- 5x 10/100 Mbps Ethernet
- 2x external, non-detachable antennas
- USB 2.0 Port
- UART (J1) header on PCB (115200 8n1)
- 8x LED, 2x button, power input switch

Flash instruction:

The only way to flash LEDE image in mr3420v5 is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.0.225/24 and tftp server.
2. Rename "lede-ramips-mt7628-tplink_tl-mr3420-v5-squashfs-tftp-recovery.bin"
   to "tp_recovery.bin" and place it in tftp server directory.
3. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds, until
   device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.

Signed-off-by: Henryk Heisig <hyniu@o2.pl>