ath79: add support for ZTE MF281

ath79: add support for ZTE MF281

Add support for the ZTE MF281 battery-powered WiFi router.

Hardware
--------
SoC:    Qualcomm Atheros QCA9563
RAM:    128M DDR2
FLASH:  2M SPI-NOR (GigaDevice GD25Q16)
        128M SPI-NAND (GigaDevice)
WLAN:   QCA9563 2T2R 802.11 abgn
        QCA9886 2T2R 802.11 nac
WWAN:   ASRMicro ASR1826
ETH:    Qualcomm Atheros QCA8337
UART:   115200 8n1
        Unpopulated connector next to SIM slot
        (SIM) GND - RX - TX - 3V3
        Don't connect 3V3
BUTTON: Reset - WPS
LED:    1x debug-LED (internal)
        LEDs on front of the device are controlled
        using the modem CPU and can not be controlled
        by OpenWrt

Installation
------------

1. Connect to the serial console. Power up the device and interrupt
   autoboot when prompted

2. Connect a TFTP server reachable at 192.168.1.66 to the ethernet port.
   Serve the OpenWrt initramfs image as "speedbox-2.bin"

3. Boot the initramfs image using U-Boot

   $ setenv serverip 192.168.1.66
   $ setenv ipaddr 192.168.1.154
   $ tftpboot 0x84000000 speedbox-2.bin
   $ bootm

4. Copy the OpenWrt factory image to the device using scp and write to
   the NAND flash

   $ mtd write /path/to/openwrt/factory.bin firmware

WWAN
----

The WWAN card can be used with OpenWrt. Example configuration for
connection with a unauthenticated dual-stack APN:

network.lte=interface
network.lte.proto='ncm'
network.lte.device='/dev/ttyACM0'
network.lte.pdptype='IPV4V6'
network.lte.apn='internet.telekom'
network.lte.ipv6='auto'
network.lte.delay='10'

The WWAN card is running a modified version of OpenWrt and handles
power-management as well as the LED controller (AW9523). A root shell
can be acquired by installing adb using opkg and executing "adb shell".

Signed-off-by: David Bauer <mail@david-bauer.net>