ath79: add support for TP-Link Deco M4R v1 and v2

ath79: add support for TP-Link Deco M4R v1 and v2

This commit adds support for the TP-Link Deco M4R (it can also be M4,
TP-Link uses both names) v1 and v2. It is similar hardware-wise to the
Archer C6 v2. Software-wise it is very different. V2 has a bit different
layout from V1 but the chips are the same and the OEM firmware is the same
for both versions.

Specifications:
SoC: QCA9563-AL3A
RAM: Zentel A3R1GE40JBF
Wireless 2.4GHz: QCA9563-AL3A (main SoC)
Wireless 5GHz: QCA9886
Ethernet Switch: QCA8337N-AL3C
Flash: 16 MB SPI NOR

Flashing:

The device's bootloader only accepts images that are signed using
TP-Link's RSA key, therefore this way of flashing is not possible. The
device has a web GUI that should be accessible after setting up the device
using the app (it requires the app to set it up first because the web GUI
asks for the TP-Link account password) but for unknown reasons, the web
GUI also refuses custom images.

There is a debug firmware image that has been shared on the device's
OpenWrt forum thread that has telnet unlocked, which the bootloader will
accept because it is signed. It can be used to transfer an OpenWrt image
file over to the device and then be used with mtd to flash the device.

Pre-requisites:

- Debug firmware.
- A way of transferring the file to the router, you can use an FTP server
  as an example.
- Set a static IP of 192.168.0.2/255.255.255.0 on your computer.
- OpenWrt image.

Installation:

- Unplug your router and turn it upside down. Using a long and thin object
  like a SIM unlock tool, press and hold the reset button on the router and
  replug it. Keep holding it until the LED flashes yellow.
- Open 192.168.0.1. You should see the bootloader recovery's webpage.
  Choose the debug firmware that you downloaded and flash it. Wait until the
  router reboots (at this stage you can remove the static IP).

- Open a terminal window and connect to the router via telnet (the primary
  router should have a 192.168.0.1 IP address, secondary routers are
  different).
- Transfer the file over to the router, you can use curl to download it
  from the internet (use the insecure flag and make sure your source accepts
  insecure downloads) or from an FTP server.
- The router's default mtd partition scheme has kernel and rootfs
  separated. We can use dd to split the OpenWrt image file and flash it with
  mtd:

   dd if=openwrt.bin of=kernel.bin skip=0 count=8192 bs=256
   dd if=openwrt.bin of=rootfs.bin skip=8192 bs=256

- Once the images are ready, you have to flash the device using mtd
  (make sure to flash the correct partitions or you may be left with a
  hard bricked router):

   mtd write kernel.bin kernel
   mtd write rootfs.bin rootfs

- Flashing is done, reboot the device now.

Signed-off-by: Foica David <superh552@gmail.com>
(cherry picked from commit 063e9047cc8b247ea4b04ee3248b99f3212a42f8)