ramips-mt7621: add Xiaomi Mi Router 4A (Gigabit Edition)

Specifications:

- SoC:      MediaTek MT7621
- Flash:    16 MiB NOR SPI
- RAM:      128 MiB DDR3
- Ethernet: 3x 10/100/1000 Mbps (switched, 2xLAN + WAN)
- WIFI0:    MT7603E 2.4GHz 802.11b/g/n
- WIFI1:    MT7612E 5GHz 802.11ac
- Antennas: 4x external (2 per radio), non-detachable
- LEDs:     Programmable "power" LED (two-coloured, yellow/blue)
            Non-programmable "internet" LED (shows WAN activity)
- Buttons:  Reset

Installation:

Bootloader won't accept any serial input unless "boot_wait" u-boot
environment variable is changed to "on".

Vendor firmware won't accept any serial input until "uart_en" is
set to "1".

Using the https://github.com/acecilia/OpenWRTInvasion exploit you
can gain access to shell to enable these options:

To enable uart keyboard actions - 'nvram set uart_en=1'
To make uboot delay boot work - 'nvram set boot_wait=on'
Set boot delay to 5 - 'nvram set bootdelay=5'

Then run 'nvram commit' to make the changes permanent.

Once in the shell (following the OpenWRTInvasion instructions) you
can then run the following to flash OpenWrt and then reboot:

'cd /tmp; curl https://downloads.openwrt.org/...-sysupgrade.bin
  --output firmware.bin; mtd -e OS1 -r write firmware.bin OS1'

docs/user/supported_devices.rst

@@ -199,6 +199,10 @@ ramips-mt7621
 
   - WG3526-16M
   - WG3526-32M
+  
+* Xiaomi
+
+  - Xiaomi Mi Router 4A (Gigabit Edition)
 
 ramips-mt76x8
 -------------

targets/ramips-mt7621

@@ -29,6 +29,13 @@ device('netgear-wndr3700-v5', 'netgear_wndr3700-v5', {
 })
 
 
+-- Xiaomi
+
+device('xiaomi-mi-router-4a-gigabit-edition', 'xiaomi_mi-router-4a-gigabit', {
+	factory = false,
+})
+
+
 -- ZBT
 
 device('zbtlink-zbt-wg3526-16m', 'zbtlink_zbt-wg3526-16m', {