ramips: add support for SNR-CPE-ME2-SFP

ramips: add support for SNR-CPE-ME2-SFP

SNR-CPE-ME2-SFP is a wireless router with SFP cage manufactured by SNR/NAG company.

Specification:
- SoC: MediaTek MT7621A
- CPU: 880MHz
- Flash: 16 MB (GD25Q127CSIG)
- RAM:  256 MB
- WLAN: 2.4 GHz, 5 GHz (MediaTek MT7615DN)
- Ethernet: 4x 10/100/1000 Mbps
- SFP cage (using RTL8211FS-CG)
- USB 3.0 port
- Power: 12 VDC, 2 A

Flash instruction via TFTP:
1. Boot SNR-CPE-ME2 to recovery mode
  (press reset button and power on device, hold button for ~10 seconds)
2. Send firmware via TFTP client:
 TFTP Server address: 192.168.1.1
 TFTP Client address: 192.168.1.131
3. Wait ~120 seconds to complete flashing
4. Do sysupgrade using web-interface

MAC Addresses(stock)
--------------------
+----------+------------------+-------------------+
| use      | address          | example           |
+----------+------------------+-------------------+
| Device   | label            | 6A:C4:DD:xx:xx:28 |
| Ethernet | + 1              | 6A:C4:DD:xx:xx:29 |
| 2g       | + 2              | 6A:C4:DD:xx:xx:2A |
| 5g       | + 3              | 6A:C4:DD:xx:xx:2B |
+----------+------------------+-------------------+

Notes:
- Reading sfp eeprom is not supported [1] (driver issue). Stock image has the same situation.

References:
1. https://forum.openwrt.org/t/mt7621-and-reading-sfp-eeprom/152249

Signed-off-by: Aleksey Nasibulin <alealexpro100@ya.ru>