uboot-mediatek: add support for Bananapi BPi-R3

uboot-mediatek: add support for Bananapi BPi-R3

The Bananapi BPi-R3 board can boot from eMMC, SD card, SPI-NAND and
SPI-NOR, depending on the position of switches controlling the BOOTSEL
bootstrap pins as we as hard-wired chip-select lines. The position of the
chip-select switch SW6 decides whether either SD card or eMMC can be
accessed, SW5 selects either SPI-NAND or SPI-NOR.

Generate U-Boot for all 4 boot options. The SD card version allows
installation to SPI-NAND and SPI-NOR (eMMC cannot be accessed
simultanously with the SD card), the SPI-NAND version allows installation
to eMMC.

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