ahci: mediatek: add ahci driver

add AHCI driver ported from linux

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/ata/ahci_mtk.c

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
diff --git a/drivers/ata/Makefile b/drivers/ata/Makefile
index a69edb1..98fb480 100644
--- a/drivers/ata/Makefile
+++ b/drivers/ata/Makefile
@@ -19,3 +19,4 @@
 obj-$(CONFIG_SANDBOX) += sata_sandbox.o
 obj-$(CONFIG_AHCI_MVEBU) += ahci_mvebu.o
 obj-$(CONFIG_SUNXI_AHCI) += ahci_sunxi.o
+obj-$(CONFIG_MTK_AHCI) += mtk_ahci.o