mediatek: add support for BananaPi BPI-R4 board

mediatek: add support for BananaPi BPI-R4 board

Hardware
--------
SOC:    MediaTek MT7988A (4x Cortex-A73)
RAM:    4 GiB DDR4
Flash:  128 MiB Winbond SPI-NAND
MMC:    8 GiB eMMC *or* microSD (cannot be used both)
ETH:    4x 1GE (1x WAN, 3x LAN)
        2x SFP+ (10G, 5G, 2.5G, 1G)
USB:    on-board USB 3.2 4-port hub
        1x USB 3.2 port (type A connector)
        1x M.2 for 4G/5G modem
        2x mPCIe for additional modems
WiFi:   optional MediaTek MT7996 Wi-Fi 7 module
        (using 2x PCIe gen3 x2 on the mPCIe slots and 12V power)

Installation
------------
1. Decompress and write the sdcard image to a micro SD card and use that
to boot the R4 (both dip switches in upper position).

2. Use the bootloader menu accessible via the serial console to install
to SPI-NAND.

3. Switch to boot from SPI-NAND and install to eMMC.

Known issues
------------
 - The RST button is hard-wired to the SoC reset and can't be read
   from software. This can be changed by modifying the board (ie.
   moving a 0-ohm resistor). However, in order to maintain compatibility
   with the board as it comes from factory the button isn't used by
   OpenWrt and the WPS button is used as factory/reset button instead.

 - various small things still need to be fixed in DT

Signed-off-by: Daniel Golle <daniel@makrotopia.org>