mediatek: Add support for TP-Link EAP225v5

mediatek: Add support for TP-Link EAP225v5

Device specification:
- MT7629 with 16MB NOR flash W25Q128 and 128 MB DDR3 RAM.
- MT7761N and MT7762N wireless chips (currenlty no driver in OpenWrt available)
  - WiFi is NOT working on this device
- Dual core but second CPU doesn't seem to work (Error message during boot: "CPU1: failed to come online")

There are two similar merge requests for similar devices with the same issues:
- https://github.com/openwrt/openwrt/pull/12286
- https://github.com/openwrt/openwrt/pull/5084

UART interface is next to the reset button, pinout:
- 1: TX (the pin with the arrow marker)
- 2: RX
- 3: GND
- 4: VCC

UART settings: 115200,8n1, 3.3V

U-Boot menu can be entered by pressing Ctrl+B during startup.

Booting initramfs:
- Set your computers IP adress to 192.168.1.110
- Run a TFTP server providing the initramfs image
- Power on the AP, press Ctrl+B to get to the U-Boot menu
- Select "1. System Load Linux to SDRAM via TFTP"
- Update kernel file name, input server IP and input device IP (if they deviate from the defaults)
- After booting, create a backup of all partitions, especially for kernel and root_fs. They are required for reverting back to stock firmware
- The sysupgrade image can be flashed now

MAC adresses:
- LAN and 2.4GHz use the same MAC (the one printed on the device)
- 5GHz WiFi MAC is LAN MAC + 1

GPIOs:
- GPIO 21 is the reset pin (low active)
- GPIO 55 is for the green LED (active high)
- GPIO 56 is for the yellow/amber LED (active high)

Signed-off-by: Roland Reinl <reinlroland+github@gmail.com>
(cherry picked from commit 44cd32d764fe1daed3b6d0595530df0c4d1c3464)