board: mediatek: add MT7988 reference boards

This patch adds general board files based on MT7988 SoCs.

MT7988 uses one mmc controller for booting from both SD and eMMC,
and the pins of mmc controller booting from SD are also shared with
one of spi controllers.
So two configs are need for these boot types:

1. mt7988_rfb_defconfig - SPI-NOR, SPI-NAND and eMMC
2. mt7988_sd_rfb_defconfig - SPI-NAND and SD

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>