ramips: add support for Archer C50 v4

This adds support for the TP-Link Archer C50 v4.
It uses the same hardware as the v3 variant, sharing the same FCC-ID.

CPU:   MediaTek MT7628 (580MHz)
RAM:   64M DDR2
FLASH: 8M SPI
WiFi:  2.4GHz 2x2 MT7628 b/g/n integrated
WiFI:  5GHz 2x2 MT7612 a/n/ac
ETH:   1x WAN 4x LAN
LED:   Power, WiFi2, WiFi5, LAN, WAN, WPS
BTN:   WPS/WiFi, RESET
UART:  Near ETH ports, 115200 8n1, TP-Link pinout

Create Factory image
--------------------
As all installation methods require a U-Boot to be integrated into the
Image (and we do not ship one with the image) we are not able to create
an image in the OpenWRT build-process.

Download a TP-Link image from their Wesite and a OpenWRT sysupgrade
image for the device and build yourself a factory image like following:

TP-Link image:             tpl.bin
OpenWRT sysupgrade image:  owrt.bin

 > dd if=tpl.bin of=boot.bin bs=131584 count=1
 > cat owrt.bin >> boot.bin

Installing via Web-UI
---------------------
Upload the boot.bin via TP-Links firmware upgrade tool in the
web-interface.

Installing via Recovery
-----------------------
Activate Web-Recovery by beginning the upgrade Process with a
Firmware-Image from TP-Link. After starting the Firmware Upgrade,
wait ~3 seconds (When update status is switching to 0%), then
disconnect the power supply from the device. Upgrade flag (which
activates Web-Recovery) is written before the OS-image is touched and
removed after write is succesfull, so this procedure should be safe.

Plug the power back in. It will come up in Recovery-Mode on 192.168.0.1.
When active, all LEDs but the WPS LED are off.
Remeber to assign yourself a static IP-address as DHCP is not active in
this mode.

The boot.bin can now be uploaded and flashed using the web-recovery.

Installing via TFTP
-------------------
Prepare an image like following (Filenames from factory image steps
apply here)

 > dd if=/dev/zero of=tp_recovery.bin bs=196608 count=1
 > dd if=tpl.bin of=tmp.bin bs=131584 count=1
 > dd if=tmp.bin of=boot.bin bs=512 skip=1
 > cat boot.bin >> tp_recovery.bin
 > cat owrt.bin >> tp_recovery.bin

Place tp_recovery.bin in root directory of TFTP server and listen on
192.168.0.66/24.

Connect router LAN ports with your computer and power up the router
while pressing the reset button. The router will download the image via
tftp and after ~1 Minute reboot into OpenWRT.

U-Boot CLI
----------
U-Boot CLI can be activated by holding down '4' on bootup.

Dual U-Boot
-----------
This is the first TP-Link MediaTek device to feature a split-uboot
design. The first (factory-uboot) provides recovery via TFTP and HTTP,
jumping straight into the second (firmware-uboot) if no recovery needs
to be performed. The firmware-uboot unpacks and executed the kernel.

Web-Recovery
------------
TP-Link integrated a new Web-Recovery like the one on the Archer C7v4 /
TL-WR1043v5. Stock-firmware sets a flag in the "romfile" partition
before beginning to write and removes it afterwards. If the router boots
with this flag set, bootloader will automatically start Web-recovery and
listens on 192.168.0.1. This way, the vendor-firmware or an OpenWRT
factory image can be written.

By doing the same while performing sysupgrade, we can take advantage of
the Web-recovery in OpenWRT.

It is important to note that Web-Recovery is only based on this flag. It
can't detect e.g. a crashing kernel or other means. Once activated it
won't boot the OS before a recovery action (either via TFTP or HTTP) is
performed. This recovery-mode is indicated by an illuminated WPS-LED on
boot.

Signed-off-by: David Bauer <mail@david-bauer.net>

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