ramips: add Xiaomi Mi Router 4A Gigabit explicitly

ramips: add Xiaomi Mi Router 4A Gigabit explicitly

This device has previously been supported by the image
for Xiaomi Mi Router 3G v2. Since this is not obvious, the
4A is marketed as a new major revision and it also seems to
have a different bootloader, this will be both more tidy and
more helpful for the users.

Apart from that, note that there also is a 100M version of
the device that uses mt7628 platform, so a specifically named
image will also prevent confusion in this area.

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'

Suggested-by: David Bentham <db260179@gmail.com>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>