ramips: mt7621: add support for Confiabits MT7621 v1

Hardware:
 - SoC: MediaTek MT7621DAT
 - Flash: 16 MiB cFeon
 - RAM: 128 MiB MT7621DAT
 - WLAN: 2.4 GHz (MT7603EN), 5 GHz (MT7613BEN 802.11ac)
 - Ethernet: 1x 10/100/1000 Mbps WAN, 3x 10/100/1000 LAN (MT7621DAT)
 - Buttons: 1 Reset button, 1 WPS button
 - LEDs: 5x Green (POWER/WAN/2.4G/5G/WPS), 1x Red (WAN)
 - Serial console: unpopulated header, 57600 8n1
 - Power: 12 VDC, 1 A

MAC:
LAN MAC: label mac (eeprom@4)
WAN MAC: label mac (eeprom@4)
2.4G MAC: label mac (eeprom@4)
5G MAC: label mac + 1 (eeprom@8004)

Installation:
The stock firmware is OpenWrt-based. If you can reach LuCI or SSH, just use the sysupgrade image
with the 'Keep settings' option turned off.

Signed-off-by: Luis Mita <luis@luismita.com>
Link: https://github.com/openwrt/openwrt/pull/17534
(cherry picked from commit 1c5ca24d)
Link: https://github.com/openwrt/openwrt/pull/17653


Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>