ramips: support TP-Link EAP615-Wall

ramips: support TP-Link EAP615-Wall

Add support for the TP-Link EAP615-Wall, an AX1800 Wall Plate WiFi 6 AP.
The device is very similar to the TP-Link EAP235-Wall.

Hardware:
* SoC: MediaTek MT7621AT
* RAM: 128MiB
* Flash: 16MiB SPI-NOR
* Ethernet: 4x GbE
  * Back: ETH0 (PoE-PD)
  * Bottom: ETH1, ETH2, ETH3 (PoE passthrough)
* WiFi: MT7905DAN/MT7975DN 2.4/5 GHz 2T2R
* LEDS: 1x white
* Buttons: 1x LED, 1x reset

Stock firmware uses a random MAC address for ethernet. OpenWrt uses the
MAC address that is on the device label for ethernet and the wireless
interfaces. MAC address must not be incremented, as this will cause MAC
address conflicts in case you have two devices with consecutive MAC
addresses. Instead, different locally administered addresses will be
generated automatically, based on the MAC on the label.

Installation via stock firmware:
* Enable SSH in the TP-Link web interface
* SSH to the device
* Run `cliclientd stopcs`
* Upload the OpenWrt factory image via the TP-Link web interface

Installation via bootloader:
* Solder TTL header. Pinout: 1: TX, 2: RX, 3: GND, 4: VCC, with pin 1
  closest to ETH1. Baud rate 115200
* Interrupt boot process by holding a key during boot
* Boot the OpenWrt initramfs:
  # tftpboot 0x84000000 openwrt-ramips-mt7621-tplink_eap615-wall-v1-initramfs-kernel.bin
  # bootm
* Copy openwrt-ramips-mt7621-tplink_eap615-wall-v1-squashfs-sysupgrade.bin
  to /tmp and use sysupgrade to install it

Thanks to Sander Vanheule for his work on the EAP235-Wall, which made
adding support for the EAP615-Wall very easy.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Reviewed-by: Sander Vanheule <sander@svanheule.net>
Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>