ramips: Add support for ZBT WE826-E

ramips: Add support for ZBT WE826-E

ZBT WE826-E is a dual-SIM version of the ZBT WE826. The router has the
following specifications:

- MT7620A (580 MHz)
- 128MB RAM
- 32MB of flash (SPI NOR)
- 5x 10/100Mbps Ethernet (MT7620A built-in switch)
- 1x microSD slot
- 1x miniPCIe slot (only USB2.0 bus)
- 2x SIM card slots (standard size)
- 1x USB2.0 port
- 1x 2.4GHz wifi (rt2800)
- 10x LEDs (4 GPIO-controlled)
- 1x reset button

The following have been tested and working:
- Ethernet switch
- wifi
- miniPCIe slot
- USB port
- microSD slot
- sysupgrade
- reset button

Installation and recovery:

In order to install OpenWRT the first time or 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.

How to swap SIMs:

You control which SIM slot to use by writing 0/1 to
/sys/class/gpio/gpio13/value. In order for the change to take effect,
you can either use AT-commands (AT+CFUN) or power-cycle the modem (write
0/1 to /sys/class/gpio/gpio14/value).

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