ipq40xx: add support for Huawei AP4050DN

ipq40xx: add support for Huawei AP4050DN

Hardware
========
SoC: Qualcomm IPQ4018
ETH: Qualcomm QCA8075 (2 x RJ-45)
WDG: OnSemi SCT706
RAM: Micron MT41K128M16JT-125 (256MB)
NOR: Infineon S25FL512S (64MB)

Installation
============

1. Create a ramboot-able image with the command

   { dd if=/dev/zero bs=32 count=1 2>/dev/null; \
     cat openwrt-ipq40xx-generic-huawei_ap4050dn-initramfs-uImage.itb; \
   } > ramboot.bin`

2. Start a TFTP server in the folder with the ramboot.bin.

3. Plug in a USB-RJ45 serial adapter to the CONSOLE port of the device
   and start a serial console session with
   9600 baud, no parity, 1 stop bit.

4. Plug in either 12V power or PoE to the device.

5. On the prompt `Press f or F  to stop Auto-Boot in 3 seconds`,
   press `f` to enter the Huawei U-Boot command line

6. Enter a new password for the u-boot command line

7. In the command line, run these commands to ramboot OpenWrt:

      setenv serverip <IPv4 address of TFTP server>
      setenv ipaddr <IPv4 address for this AP>
      setenv rambootfile ramboot.bin
      run ramboot

8. In OpenWrt, set up the network and then `scp` the files
   `u-boot-huawei_ap4050dn/uImage` and
   `openwrt-ipq40xx-generic-huawei_ap4050dn-squashfs-sysupgrade.bin`
   into `/tmp/`

9. To backup the original firmware, run the following:

      cat /dev/mtd12 /dev/mtd13 > huawei_ap4050dn_fw_backup.bin.bin

10. Run the following commands to flash u-boot and OpenWrt to the device:

      mtd erase uboot
      mtd write /tmp/uImage uboot
      sysupgrade -n /tmp/openwrt-ipq40xx-generic-huawei_ap4050dn-squashfs-sysupgrade.bin

11. The device should now boot OpenWrt! (sometimes the boot process takes a bit
    longer due to the watchdog resetting the device before the watchdog driver runs)

Signed-off-by: Marco von Rosenberg <marcovr@selfnet.de>