ARM: MediaTek: add basic support for MT8518 boards

This adds a general board file based on MT8518 SoCs from MediaTek.

Apart from the generic parts (cpu) we add some low level init codes
and initialize the early clocks.

This commit is adding the basic boot support for the MT8518 eMMC board.

Signed-off-by: mingming lee <mingming.lee@mediatek.com>
[trini: Migrate env location to defconfig, set ENV_IS_IN_MMC]
Signeed-off-by: Tom Rini <trini@konsulko.com>