ramips: add support for ZBT WE1026-H

ramips: add support for ZBT WE1026-H

This commit adds support for the ZBT WE1026-H, an outdoor AP with
support for adding an internal LTE modem. The detailed specs are:

* CPU: MT7620A
* 2x 10/100Mbps Ethernet (LAN port has passive PoE support).
* 16/32 MB Flash.
* 128/256 MB RAM.
* 1x USB 2.0 port.
* 1x mini-PCIe slot (only USB2.0 bus).
* 1x SIM slot (standard size).
* 1x 2.4Ghz WIFI (rt2800).
* 1x button.
* 6x LEDS (4 GPIO-controlled).
* 1x micro-SD reader.

The following have been tested and working:
- Ethernet switch
- Wifi
- Mini-PCIe slot + SIM slot
- USB port
- microSD slot
- sysupgrade
- reset button

Installation and recovery:

In order to install OpenWRT the first time or ito recover the router,
you can use the web-based recovery system. Keep the reset button pressed
during boot and access 192.168.1.1 in your browser when your machine
obtains an IP address. Upload the firmware to start the recovery
process.

Notes:

* When binding the USB LED to a usbport, the LED is switched on all the
time due to the presence of an internal hub. Thus, it does not really
signal any USB-information.

* I only have the 32MB version and have only added support for this
device. However, the files are structured so that adding support for the
16MB version should be easy.

* Only the LAN port is accessible from the outside of the casing and LEDs
are not visible.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
[rebased onto base-files split, minor style fixes, removed use of
USB led as power LED]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>