This adds a general board file based on MT7623 SoCs from MediaTek.

As this u-boot is loaded by MTK proprietary preloader, there is no
low level initializtion codes.
Signed-off-by: NWeijie Gao <weijie.gao@mediatek.com>
Signed-off-by: NRyder Lee <ryder.lee@mediatek.com>
Tested-by: NMatthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>