This patch adds general board files based on MT7981 SoCs.
MT7981 uses one mmc controller for booting from both SD and eMMC, and the
pins of mmc controller are also shared with spi controller.
So three configs are need for these boot types:
1. mt7981_rfb_defconfig - SPI-NOR and SPI-NAND
2. mt7981_emmc_rfb_defconfig - eMMC only
3. mt7981_sd_rfb_defconfig - SD only
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>