ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/omap5_uevm_defconfig b/configs/omap5_uevm_defconfig
index acf8962..b3c6fa9 100644
--- a/configs/omap5_uevm_defconfig
+++ b/configs/omap5_uevm_defconfig
@@ -33,6 +33,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ISO_PARTITION=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_SCSI_AHCI=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y
 CONFIG_CMD_TCA642X=y