ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/highbank_defconfig b/configs/highbank_defconfig
index 20a7b71..f55b5dc 100644
--- a/configs/highbank_defconfig
+++ b/configs/highbank_defconfig
@@ -23,6 +23,7 @@
 CONFIG_EFI_PARTITION=y
 # CONFIG_PARTITION_UUIDS is not set
 CONFIG_ENV_IS_IN_NVRAM=y
+CONFIG_SCSI_AHCI=y
 # CONFIG_MMC is not set
 CONFIG_SCSI=y
 CONFIG_OF_LIBFDT=y