ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/MPC8544DS_defconfig b/configs/MPC8544DS_defconfig
index 6e30ac4..4f9219b 100644
--- a/configs/MPC8544DS_defconfig
+++ b/configs/MPC8544DS_defconfig
@@ -18,6 +18,7 @@
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
 CONFIG_ENV_IS_IN_FLASH=y
+CONFIG_SCSI_AHCI=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_PHYLIB=y