ramips: Add support for ZTE ZXECS EBG3130 aka BDCOM WAP2100-SK

ramips: Add support for ZTE ZXECS EBG3130 aka BDCOM WAP2100-SK

On the bottom sticker it's branded as ZTE ZXECS EBG3130 device, but in factory
OpenWrt image it's referenced as BDCOM WAP2100-SK device.

Specifications:

- SoC: MediaTek MT7620A
- RAM: 128 MB
- Flash: 16 MB
- Ethernet: 5 FE ports
- Wireless radio: 2T2R 2.4 GHz and 1T1R 5 GHz (MT7610EN, unsupported)
- UART: 1 x UART on PCB marked as J2 (R=RX, T=TX, G=GND) with 115200 8N1 config
- LEDs: Power, FE ports 1-5, WPS, USB, RF 2.4G, RF 5G
- Other: USB port, SD card slot and 2x external antennas (non-detachable)

Flashing instructions:

A) The U-Boot has HTTP based firmware upgrade

  A1) Flashing notes

  We've identified so far two different batches of units, unfortunately
  each batch has different U-Boot bootloader flashed with different
  default environment variables, thus each batch has different IP address
  for accessing web based firmware updater.

  * First batch has web based bootloader IP address 1.1.1.1
  * Second batch has web based bootloader IP address 192.168.1.250

  In case you can't connect to either of those IPs, you can try to get
  the default IP address via two methods:

  A1.1) Serial console, then the IP address is visible during the boot

   ...
   HTTP server is starting at IP: 1.1.1.1
   raspi_read: from:40004 len:6
   HTTP server is ready!
   ...

  A1.2) Over telnet/SSH using this command:

   root@bdcom:/# grep ipaddr= /dev/mtd0
   ipaddr=1.1.1.1

  A2) Flashing with browser

  * Change IP address of PC to 1.1.1.2 with 255.255.255.0 netmask
  * Reboot the device and try to reach web based bootloader in the
    browser with the following URL http://1.1.1.1

  * Quickly select the firmware sysupgrade file and click on the
    `Update firmware` button, this all has to be done within 10 seconds,
    bootloader doesn't wait any longer

   If done correctly, the web page should show UPDATE IN PROGRESS page
   with progress indicator. Once the flashing completes (it takes roughly
   around 1 minute), the device will reboot to the OpenWrt firmware

  A3) Flashing with curl

   sudo ip addr add 1.1.1.2/24 dev eth0
   curl \
      --verbose \
      --retry 3 \
      --retry-delay 1 \
      --retry-max-time 30 \
      --connect-timeout 30 \
      --form "firmware=@openwrt-ramips-mt7620-BDCOM-WAP2100-SK-squashfs-sysupgrade.bin" \
      http://1.1.1.1

   Now power on the router.

B) The U-boot is based on Ralink SDK so we can flash the firmware using UART.

   1. Configure PC with a static IP address and setup an TFTP server.
   2. Put the firmware into the tftp directory.
   3. Connect the UART line as described on the PCB (G=GND, R=RX, T=TX)
   4. Power up the device and press 2, follow the instruction to set device and
      tftp server IP address and input the firmware file name. U-boot will then load
      the firmware and write it into the flash.

Signed-off-by: Petr Štetiar <ynezz@true.cz>