ata: mediatek: add support for MediaTek SATA controller

This adds support the AHCI-compliant Serial ATA controller present
on MediaTek SoCs.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>