ramips: add support for netis N6

ramips: add support for netis N6

This commit adds support for netis N6 WiFi 6 router.

Specification
-------------
- SoC       : MediaTek MT7621AT, MIPS, 880 MHz
- RAM       : 256 MiB
- Flash     : NAND 128 MiB (ESMT PSU1GA30DT)
- WLAN      : MT7905DAN + MT7975DN
  - 2.4 GHz : b/g/n/ax, 574 Mbps, MIMO 2x2
  - 5 GHz   : a/n/ac/ax, 1201 Mbps, MIMO 2x2
- Ethernet  : 10/100/1000 Mbps x5 (1x WAN, 4x LAN)
- USB       : 1x 3.0
- UART      : 3.3V, 115200n8
- Buttons   : 1x Reset
              1x WPS
- LEDs      : 1x Power (green)
              1x System (green)
              1x WAN (green)
              1x WiFi 2.4 GHz (green), controlled by phy
              1x WiFi 5 GHz (green), controlled by phy
              1x WPS (green)
              1x USB (green)
              5x ethernet leds (green), controlled by switch
- Power     : 12 VDC, 1.5 A

Installation
------------
1. Update the router using stock firmware web interface and OpenWrt
   factory.bin image.

Recovery and return to stock
----------------------------
1. Assign your PC a static IP 192.168.1.2 and connect to the router using
   the ethernet cable;
2. Power off the router;
3. Press Reset button, power on the router and wait until ethernet led
   start blinking;
4. Release the button;
5. Open http://192.168.1.1/ (N6 System Recovery Mode) in your browser;
6. Upload OpenWrt factory.bin (or stock firmware *.bin) image and proceed
   with upgrade.

MAC addresses
-------------
+---------+-------------------+
|         | MAC example       |
+---------+-------------------+
| LAN     | dc:xx:xx:49:xx:04 |
| WAN     | dc:xx:xx:49:xx:05 |
| WLAN 2g | dc:xx:xx:19:xx:06 |
| WLAN 5g | dc:xx:xx:79:xx:06 |
+---------+-------------------+
The WLAN MAC prototype was found in 'Factory', 0x4
The LAN MAC was found in 'Factory', 0x7ef20
The WAN MAC was found in 'Factory', 0x7ef26

Known issue
-----------
2.4 GHz WLAN doesn't start with mt76 driver.

Probable reason:
   Original Netis N6 EEPROM contains wrong MT_EE_WIFI_CONF value (0xd2).
   Other routers with the same WLAN hardware (e.g., Routerich AX1800)
   have MT_EE_WIFI_CONF = 0x92.

Workaround (already included in this commit):
   Extract EEPROM to a file at the first time boot and change
   MT_EE_WIFI_CONF (offset 0x190) value from 0xd2 to 0x92. See
   /etc/hotplug.d/firmware/11-mt76-caldata for details.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16322
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>