ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/Cubieboard_defconfig b/configs/Cubieboard_defconfig
index c670ab8..8c1c133 100644
--- a/configs/Cubieboard_defconfig
+++ b/configs/Cubieboard_defconfig
@@ -14,6 +14,7 @@
 # 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_SCSI=y
 CONFIG_USB_EHCI_HCD=y