mediatek: add support for Cetron CT3003

mediatek: add support for Cetron CT3003

**Hardware specification:**

- SoC: MediaTek MT7981B 2x A53
- Flash: ESMT F50L1G41LB 128MB
- RAM: Nanya NT5CC128M16JR-EK 256MB
- Ethernet: 4 x 10/100/1000 Mbps
- Switch: MediaTek MT7531AE
- WiFi: MediaTek MT7976C
- Button: Reset, Mesh
- Power: DC 12V 1A
- UART: 3.3v, 115200n8
  | Layout:   |
  | :-------- |
  | <Antenna> |
  | VCC       |
  | GND       |
  | Tx        |
  | Rx        |

**Flash instructions:**

1. Rename `openwrt-mediatek-filogic-cetron_ct3003-squashfs-factory.bin` to `factory.bin`.
2. Upload the `factory.bin` using the device's Web interface.
3. Click the upgrade button and wait for the process to finish.
4. Access the OpenWrt interface using the same password.
5. Use the 'Restore' function to reset the firmware to its initial state.

**Notes:**

If you plan to recovery the stock firmware in the future, it's advisable
to connect the device via the serial port and enter failsafe mode to
back up all the MTD partitions before proceeding the steps above.

Signed-off-by: Patricia Lee <patricialee320@gmail.com>