ramips: Add support for ZBT WE1026-5G

ramips: Add support for ZBT WE1026-5G

The ZBT WE1026-5G
(http://www.zbtlink.com/products/router/WE1026-5G.html) is the follow-up
to the ZBT WE1026 and is based on MT7620. For the previous WE1026, the
ZBT WE826 image could be used. However, as the name implies, the -5G
comes equipped with a 5GHz wifi radio. As the WE826 only has a 2.4GHz
radio, the addition of 5GHz means that a separate image is needed for
the WE1026-5G. I suspect that this image will also work on the previous
WE1026, but I don't have a device to test with.

The WE1026-5G has following specifications:
* CPU: MT7620A
* 1x 10/100Mbps Ethernet.
* 16 MB Flash.
* 64 MB RAM.
* 1x USB 2.0 port.
* 1x mini-PCIe slots.
* 1x SIM slots.
* 1x 2.4Ghz WIFI.
* 1x 5GHz wifi (MT7612)
* 1x button.
* 3x controllable LEDs.

Works:
* Wifi.
* Switch.
* mini-PCIe slot. Only tested with a USB device (a modem).
* SIM slot.
* Sysupgrade.
* Button (reset).

Not working:
* The 5GHz WIFI LED is completely dead. I suspect the issue is the same
as on other devices with Mediatek 5Ghz wifi-cards/chips. The LED is
controlled by the driver, and mt76 (currently) does not support this.

Not tested:
* SD card reader.

Notes:
* The modem (labeled 3G/4G) and power LEDs are controlled by the
hardware.
* There is a 32MB version of this device available, but I do not have
access to it. I have therefor only added support for the 16MB version,
but added all the required infrastructure to make adding support for the
32MB version easy.

Installation:
The router comes pre-installed with OpenWRT, including a variant of
Luci. The initial firmware install can be done through this UI,
following normal procedure. I.e., access the UI and update the firmware
using the sysupgrade-image. Remember to select that you do not want to
keep existing settings.

Recovery:
If you brick the device, the WE1026-5G supports recovery using HTTP. Keep the
reset button pressed for ~5sec when booting to start the web server. Set the
address of the network interface on your machine to 192.168.1.2/24, and
point your browser to 192.168.1.1 to access the recovery UI. From the
recovery UI you can upload a firmware image.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>