mediatek: add support for Buffalo WSR-2533DHP2

mediatek: add support for Buffalo WSR-2533DHP2

This adds support for the Buffalo WSR-2533DHP2.

The device uses the Broadcom TRX image format with a special magic. To
be able to boot the images or load them they have to be wrapped with
different headers depending how it is loaded.

There are multiple ways to install OpenWrt on this device.
Boot ramdisk from U-Boot
----------------------------
This will load the image and not write it into the flash.

1. Stop boot menu with "space" key
2. Select "System Load Linux to SDRAM via TFTP."
3. Load this image:
   openwrt-mediatek-mt7622-buffalo_wsr-2533dhp2-initramfs-kernel.bin
4. The system boots the image

Write to flash from U-Boot
-----------------------------
This will load the image over tftp and directly write it into the flash.

1. Stop boot menu with "space" key
2. Select "System Load Linux Kernel then write to Flash via TFTP."
3. Load this image:
   openwrt-mediatek-mt7622-buffalo_wsr-2533dhp2-squashfs-factory-uboot.bin
4. The system writes this image into the flash and boots into it.

Write to flash from Web UI
-----------------------------
This will load the image over over the Web UI and write it into the flash

1. Open the Web UI
2. Go to "管理" -> "ファームウェア更新"
3. Select "ローカルファイル指定" and click "更新実行"
4. Load this image:
   openwrt-mediatek-mt7622-buffalo_wsr-2533dhp2-squashfs-factory.bin
5. The system writes this image into the flash and boots into it.

Specifications
-------------------
* SoC:       MT7622 (4x4 2.4 GHz Wifi)
* Wifi:      MT7615 (4x4 5 GHz Wifi)
* Flash:     Winbond W29N01HZ 128MB SLC NAND
* RAM        256MB
* Ethernet:  Realtek RTL8367S (5 x 1GBit/s, SoC via 2.5GBit/s)

Co-Developed-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>