ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/MPC8572DS_defconfig b/configs/MPC8572DS_defconfig
index 9dd7c73..86546f0 100644
--- a/configs/MPC8572DS_defconfig
+++ b/configs/MPC8572DS_defconfig
@@ -17,6 +17,7 @@
 CONFIG_CMD_PING=y
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
+CONFIG_SCSI_AHCI=y
 CONFIG_SYS_FSL_DDR2=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y