ramips: add support for TP-Link EX220 v1

ramips: add support for TP-Link EX220 v1

This device is very similar, if not identical, to the TP-Link AX23 v1
but is targeted at service providers and features a completely different
flash layout.

Hardware
--------

CPU:    MediaTek MT7621 DAT
RAM:    128MB DDR3 (integrated)
FLASH:  16MB SPI-NOR
WiFi:   MediaTek MT7905 + MT7975 (2.4 / 5 DBDC) 802.11ax
SERIAL: 115200 8N1
        LEDs - (3V3 - GND - RX - TX) - ETH ports

Installation
------------

Flashing is only possible via a serial connection using the sysupgrade
image; the factory image must be signed. You can flash the sysupgrade
image directly through the U-Boot console, or preferably, by booting the
initramfs image and flashing with the sysupgrade command. Follow these
steps for sysupgrade flashing:

1. Establish a UART serial connection.
2. Set up a TFTP server at 192.168.0.2 and copy the initramfs image
   there.
3. Power on the device and press any key to interrupt normal boot.
4. Load the initramfs image using tftpboot.
5. Boot with bootm.
6. If you haven't done so already, back up all stock mtd partitions.
7. Copy the sysupgrade image to the router.
8. Flash OpenWrt through either LuCI or the sysupgrade command. Remember
   not to attempt saving settings.

Revert to stock firmware
------------------------

Flash stock firmware via OEM web-recovery mode. If you don't have access
to the stock firmware image, you will need to restore the firmware
partition backed up earlier.

Web-Recovery
------------

The router supports an HTTP recovery mode:

1. Turn off the router.
2. Press the reset button and power on the device.
3. When all LEDs start flashing, release reset and quickly press it
   again.

The interface is reachable at 192.168.0.1 and supports installation of
the OEM factory image. Note that flashing OpenWrt this way is not
possible, as mentioned above.

Signed-off-by: Darlan Pedro de Campos <darlanpedro@gmail.com>