ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/drivers/ata/Kconfig b/drivers/ata/Kconfig
index 25ef5ce..38318b4 100644
--- a/drivers/ata/Kconfig
+++ b/drivers/ata/Kconfig
@@ -20,6 +20,11 @@
 
 	  See also CMD_SATA which provides command-line support.
 
+config SCSI_AHCI
+	bool "Enable SCSI interface to SATA devices"
+	help
+	  Enable this to allow interfacing SATA devices via the SCSI layer.
+
 menu "SATA/SCSI device support"
 
 config AHCI_PCI