mediatek: add Keenetic KN-1812/Netcraze NC-1812 support

mediatek: add Keenetic KN-1812/Netcraze NC-1812 support

This commit adds support for Keenetic KN-1812/Netcraze NC-1812

Device specification
--------------------
SoC Type:   MediaTek MT7988D, Cortex-A73, 64-bit
RAM:        1024MB DDR4 Micron MT40A512M16TB-062E:R
Flash:      SPI NAND Winbond W25N02KV (256 MiB)
Ethernet:   4x1GbE (internal MT7988 built-in) + 2.5GbE (internal MT7988 phy) + 10GbE (RTL8261BE)

WLAN:       MT7992AV
WLAN 2g:    MediaTek MT7975N, b/g/n/ax/be, MIMO 4x4
WLAN 5g:    MediaTek MT7977B, a/n/ac/ax/be, MIMO 4x4
LEDs:       5 LEDs, 1 power green, 1 internet green,
            2x fn green, 1 wlan green, gpio-controlled
Button:     4 (Reset, WPS, FN1, FN2)
USB port:   Yes, 1xUSB3.2 and 1xUSB2.0 (via GL850G)
Power:      12 VDC, 3 A

Notes:
 * The device supports dual boot mode
 * Fn2 led reassigned to wlan 2.4

Flash instruction:
The only way to flash OpenWrt image is to use tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.1.2/24 and tftp server.
2. a) Keenetic
      Rename "openwrt-mediatek-filogic-keenetic_kn-1812-squashfs-factory.bin"
      to "KN-1812_recovery.bin" and place it in tftp server directory.
   b) Netcraze
      Rename "openwrt-mediatek-filogic-netcraze_nc-1812-squashfs-factory.bin"
      to "NC-1812_recovery.bin" and place it in tftp server directory.
3. Connect PC with ethernet port, press the reset button, power up
   the device and keep button pressed until status led start blinking.
4. Device will download file from server, write it to flash and reboot.

Signed-off-by: Maxim Anisimov <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20737
Signed-off-by: Hauke Mehrtens <[email protected]>