arm: MediaTek: add basic support for MT7623 boards

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.

Tested-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
12 files changed