ramips: mt76x8: add support for Wodesys WD-R1208U

ramips: mt76x8: add support for Wodesys WD-R1208U

This commit adds support for a dual-band AC1200 wall plug
manufactured by Shenzhen Century Xinyang Tech Co., Ltd.

SoC:     Mediatek MT7628AN (MIPS 24KEc single core, 580 MHz)
RAM:     128 MiB DDR2 (Hynix HY5PS1G1631C)
ROM:     8 MiB SPI NOR (Zbit ZB25VQ64ASIG)
Wired:   one FE RJ45 port (+ an unpopulated footprint for a 2nd)
WiFi:    Mediatek MT7612E
Ant.:    four 2 dBi external antennas (two 2.4GHz, two 5 GHz)
LEDs:    - Power    (green, always on)
         - 2.4G     (green, controlled by MT7628)
         - 5G       (green, controlled by MT7612)
         - Extender (green, GPIO 37, used as status LED)
         - LAN      (green/yellow, controlled by RT3050 ESW)
Buttons: WPS and reset (both connected to GPIO 38)
Power:   5V 2-pin JST-XH on main PCB
         110/220V AC to 5V 1.5A DC on auxiliary PCB
UART:    57600 8n1 3.3v, holes available on the PCB as J5
         pinout is (Gnd) (Tx) (Rx)
MAC:     1C:BF:CE:xx:xx:xx       (2.4 GHz, label)
         1C:BF:CE:xx:xx:xx + 1   (LAN)
         1C:BF:CE:xx:xx:xx + 2   (WAN, not in use)
         1C:BF:CE:xx:xx:xx + 3   (5 GHz)

Original firmware is Chaos Calmer 15.05.01 (kernel 3.10.108)
with a few custom packages and a non-LuCI web interface.
Telnet is enabled, requiring an unknown root password [1].
Root password is also needed to access the router via UART console,
but passwordless telnet can be enabled via a trivial web exploit [2]
and then the root password can be removed by editing `/etc/shadow`.

Installation: Upload `sysupgrade` binary via web interface at
  `http://192.168.188.1/settings.shtml`. Alternatively, remove
  root password and use u-boot menu to flash image via TFTP.

Notes:
- Device model in Chaos Calmer is "mtk-apsoc-demo".
- It is sold under several brands, e.g., Fenvi and Linkavenir.
  It is available in two colors: white and black.
- PCB is marked "WD206AD v1.0".
- Instead of a standard ethernet transformer, the PCB has a few tiny
  SMD coils.
- The housing is identical to the one used by a 2020 model,
  WD-R1203U, which is RTL8812-based. The older model has an FCC
  listing with external and internal images: ZNPWD-R1203U.
  The FCC listing contains a letter [3] claiming WD-R1203U and
  WD-R1208U are internally identical, but evidently they are not.

[1] root:$1$7rmMiPJj$91iv9LWhfkZE/t7aCBdo.0:18388:0:99999:7:::
    This is the same hash as in Wodesys WD-R1802U.
    There are other root password hashes in `/etc/shadow_sf` and
    `/etc/shadow_yn`.
[2] curl -X POST http://192.168.188.1/cgi-bin/adm.cgi \
    -d page=Lang -d langType="en;killall telnetd;telnetd -l /bin/sh"
[3] https://fcc.report/FCC-ID/ZNPWD-R1203U/4767033

Signed-off-by: Rani Hod <rani.hod@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19535
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>