ath79: Add support for ZBT-WD323

ath79: Add support for ZBT-WD323

ZBT-WD323 is a dual-LTE router based on AR9344. The detailed
specifications are:

* AR9344 560MHz/450MHz/225MHz (CPU/DDR/AHN).
* 128 MB RAM
* 16MB of flash(SPI-NOR, 22MHz)
* 1x 2.4GHz wifi (Atheros AR9340)
* 3x 10/100Mbos Ethernet (AR8229)
* 1x USB2.0 port
* 2x miniPCIe-slots (USB2.0 only)
* 2x SIM slots (standard size)
* 4x LEDs (1 gpio controlled)
* 1x reset button
* 1x 10 pin terminal block (RS232, RS485, 4x GPIO)
* 2x CP210x UART bridge controllers (used for RS232 and RS485)
* 1x 2 pin 5mm industrial interface (input voltage 12V~36V)
* 1x DC jack
* 1x RTC (PCF8563)

Tested:
- Ethernet switch
- Wifi
- USB port
- MiniPCIe-slots (+ SIM slots)
- Sysupgrade
- Reset button
- RS232

Intallation and recovery:

The board ships with OpenWRT, but sysupgrade does not work as a
different firmware format than what is expected is generated.  The
easiest way to install (and recover) the router, is to use the
web-interface provided by the bootloader (Breed).

While the interface is in Chinese, it is easy to use. First, in order to
access the interface, you need to hold down the reset button for around
five seconds. Then, go to 192.168.1.1 in your browser. Click on the
second item in the list on the left to access the recovery page. The
second item on the next page is where you select the firmware.  Select
the menu item containing "Atheros SDK" and "16MB" in the dropdown close
to the buttom, and click on the button at the bottom to start
installation/recovery.

Notes:
* RS232 is available on /dev/ttyUSB0 and RS485 on /dev/ttyUSB1

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
[removed unused poll-interval from gpio-keys, i2c-gpio 4.19 compat]
Signed-off-by: Petr Štetiar <ynezz@true.cz>