ram: MediaTek: add DDR3 driver for MT7629 SoC

This patch adds a DDR3 driver for MT7629 SoC.

Signed-off-by: Wu Zou <wu.zou@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>