mediatek: Add support for Linksys EA7500 v3

mediatek: Add support for Linksys EA7500 v3

Specification:
- MT7629 CPU
- MT7531 switch
- MT7761N and MT7762N wifi
- 256 MB RAM
- 128 MB NAND flash with dual-boot partitions
- 2 buttons: WPS and reset
- 1 WAN port (1G)
- 4 LAN ports (1G)
- 1 USB port

Limitations (same as other MT7629/MT7761N/MT7762N devices):
- Wifi is not working
- Second core is not working (kernel error message "CPU1: failed to come online")

Disassembly:
- There are two screws under the front rubber feet and two under the label on the bottom (in the corners towards the back, you should be able to feel them).

Serial Interface:
- UART pin header is already soldered on the board. Pinning from front to back:
1 - VCC
2 - TX
3 - RX
4 - n/a
5 - GND

GPIO:
- 1 white LED, connected to GPIO 52
- 1 reset button, connected to GPIO 60
- 1 WPS button, connected to GPIO 58

MAC Adresses:
- The MAC address printed on the device label is used for LAN and WAN
- The MAC address is stored in the devinfo partition in ASCII format (hw_mac_addr=aa:bb:cc:dd:ee)
- 2.4 GHz wifi uses MAC of the device label + 1
- 5 GHz wifi uses MAC of the device label + 2

Flashing:
- OpenWrt is only runnig in the first partition of dual boot
- To ensure to be able to go back to the factory image, flash the last OEM firmware via OEM web interface. This will ensure that the OEM firmware is present on both partitions
- Because of dual boot partitions, flashing via OEM interface is not supported
- Start a TFTP server and provide the initramfs image. Default settings:
  - Router IP: 192.168.1.1
  - TFTP server IP: 192.168.1.100
  - TFTP file name: 7531.bin
- Open the device, connect UART and select " 1. System Load Linux to SDRAM via TFTP." during startup
- Adapt the settings to your environment, if required
- After initramfs is booted, flash the sysupgrade image

Return to OEM firmware:
- Run the following commands in OpenWrt to switch to the second partition
  fw_setenv boot_part 2
  fw_setenv bootimage 2
- Reboot the device. OEM firmware will start up again

Signed-off-by: Roland Reinl <reinlroland+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16067
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>