ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/am57xx_evm_defconfig b/configs/am57xx_evm_defconfig
index 6e4d04c..fc96401 100644
--- a/configs/am57xx_evm_defconfig
+++ b/configs/am57xx_evm_defconfig
@@ -39,6 +39,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_DM=y
 CONFIG_SPL_DM=y
+CONFIG_SCSI_AHCI=y
 # CONFIG_BLK is not set
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y