ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/Marsboard_A10_defconfig b/configs/Marsboard_A10_defconfig
index 8bce411..e79c3a22 100644
--- a/configs/Marsboard_A10_defconfig
+++ b/configs/Marsboard_A10_defconfig
@@ -10,6 +10,7 @@
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_ISO_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set
+CONFIG_SCSI_AHCI=y
 CONFIG_SUN4I_EMAC=y
 CONFIG_SUNXI_NO_PMIC=y
 CONFIG_SCSI=y