Add general board files based on MT7986 SoCs.
MT7986 uses one mmc controller for booting from both SD and eMMC.
Both MT7986A and MT7986B use the same pins for spi controller.
Configs for various boot types:
1. mt7986_rfb_defconfig - SPI-NOR and SPI-NAND for MT7986A/B
2. mt7986a_bpir3_emmc_defconfig - eMMC for MT7986A only
3. mt7986a_bpir3_sd_defconfig - SD for MT7986A only
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
