ramips: add support for TP-Link Archer A6 v3

ramips: add support for TP-Link Archer A6 v3

The patch adds support for the TP-Link Archer A6 v3
The router is sold in US and India with FCC ID TE7A6V3

Specification
-------------
MediaTek MT7621 SOC
RAM:         128MB DDR3
SPI Flash:   W25Q128 (16MB)
Ethernet:    MT7530 5x 1000Base-T
WiFi 5GHz:   Mediatek MT7613BE
WiFi 2.4GHz: Mediatek MT7603E
UART/Serial: 115200 8n1

Device Configuration & Serial Port Pins
---------------------------------------
ETH Ports:    LAN4 LAN3 LAN2 LAN1 WAN
             _______________________
             |                     |
Serial Pins: |   VCC GND TXD RXD   |
             |_____________________|

LEDs:         Power Wifi2G Wifi5G LAN WAN

Build Output
------------
The build will generate following set of files
[1] openwrt-ramips-mt7621-tplink_archer-a6-v3-initramfs-kernel.bin
[2] openwrt-ramips-mt7621-tplink_archer-a6-v3-squashfs-factory.bin
[3] openwrt-ramips-mt7621-tplink_archer-a6-v3-squashfs-sysupgrade.bin

How to Use - Flashing from TP-Link Web Interface
------------------------------------------------
* Go to "Advanced/System Tools/Firmware Update".
* Click "Browse" and upload the OpenWrt factory image: factory.bin[2]
* Click the "Upgrade" button, and select "Yes" when prompted.

TFTP Booting
------------
Setup a TFTP boot server with address 192.168.0.5.
While starting U-boot press '4' key to stop autoboot.
Copy the initramfs-kernel.bin[1] to TFTP server folder, rename as test.bin
From u-boot command prompt run tftpboot followed by bootm.

Recovery
--------
Archer A6 V3 has recovery page activated if SPI booting from flash fails.
Recovery page can be activated from serial console only.
Press 'x' while u-boot is starting
Note: TFTP boot can be activated only from u-boot serial console.
Device recovery address: 192.168.0.1

Thanks to: Frankis for Randmon MAC address fix.

Signed-off-by: Vinay Patil <post2vinay@gmail.com>
[remove superfluous factory image definition, whitespacing]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit f8f8935adb2be1ebce46a8d7058c76a8d3a9bd89)