ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/cm_t54_defconfig b/configs/cm_t54_defconfig
index 69b1cbf..ca80f18 100644
--- a/configs/cm_t54_defconfig
+++ b/configs/cm_t54_defconfig
@@ -38,6 +38,7 @@
 CONFIG_ISO_PARTITION=y
 CONFIG_EFI_PARTITION=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_SCSI_AHCI=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SCSI=y
 CONFIG_SYS_NS16550=y