ramips: add support for TOZED ZLT S12 PRO

ramips: add support for TOZED ZLT S12 PRO

The TOZED ZLT S12 PRO is an AC1200 router featuring 4 Ethernet ports with a
TOZED TL70-C cellular modem which supports the NCM mode.

The stock firmware does SIM locking on the modem by stopping dialing when a
different PLMN is detected. This is not the case on OpenWrt.

Specifications:
- CPU: MediaTek MT7621AT
- RAM: 256MB DDR3
- NOR Flash: MX25L12833FM2I 16MB SPI Flash
- Wi-Fi 2.4Ghz: MT7603E
- Wi-Fi 5Ghz: MT7612E
- Switch: MT7530 4x 1Gbit Ports
- WWAN: Unisoc SL8563 based TOZED TL70-C LTE CAT6 cellular modem
- USB: 1x optional USB2.0 external port
- Switches/Buttons: WPS, Reset, Power Switch
- LEDs: Power, Wi-Fi, Data, Signal 1-5, Phone

Installation and TFTP Recovery:
- Connect to serial console.
- Boot initramfs image by choosing option 1 when U-Boot prompts.
- Install sysupgrade image via OpenWrt.

Serial Pins:
Located at the bottom right when looking from the front, right under the
Reset/WPS buttons. The pinout from the left is:
- RX
- GND
- TX
Baudrate is 115200.

When connecting from a powered off state, disconnect RX as it blocks the
boot process.

Link: http://www.sztozed.com/en/contents/58/84.html
Co-developed-by: Andre Cruz <me@1conan.com>
Signed-off-by: Andre Cruz <me@1conan.com>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>