ramips: add support for Afoundry EW-1200

ramips: add support for Afoundry EW-1200

This device features both a 2.4 and 5Ghz radio, and supports
802.11a/b/g/n/ac modes.
It has 5 Gb-Ethernet ports and a USB 3.0 host port.

It is powered by the Mediatek MT7621 SoC, and the MT7602E and MT7612E wifi
chipsets, together with 128MB of RAM and 16 MB of SPI Flash.

The stock firmware is in fact based on some openwrt barrier breaker, with a
mediatek SDK kernel, and an afoundry custom made web interface (not LuCI
based).
Firmware update page on the stock web interface can not accept sysupgrade
images, it bricks the device.
At this point, the only working solution I found was to connect to the
serial console port (available on J4 header) and to use opkg to install
dropbear.
Then scp the sysupgrade file in the device's /tmp and run sysupgrade from
console without preserving configuration files.

Signed-off-by: Francois Goudal <francois@goudal.net>