ipq806x: add support for TP-Link Archer VR2600v

ipq806x: add support for TP-Link Archer VR2600v

This router is similar to the C2600. Ethernet on WAN + LAN, switch,
sysupgrade, LEDs, buttons and WiFi on 2G + 5G do work. The xDSL modem
and the POTS/DECT interface are not supported yet.

It is not possible to flash LEDE via the TP-Link webinterface. The
image need to be signed. The first 0x200 bytes of the image is the
TP-Link header including the signature. The signature is not validated
by the bootloader. The LEDE image is zeroed in this area.

To install LEDE it is necessary to solder a four pin header to JP2.
Connect a serial interface to this header and interrupt the autostart
of kernel. Transfer the sysupgrade image via TFTP and write it to the
serial flash at 0x320000.

Signed-off-by: Sebastian Quilitz <zeraphim@x-pantion.de>