ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/cl-som-am57x_defconfig b/configs/cl-som-am57x_defconfig
index 8f0a7f6..9c3031b 100644
--- a/configs/cl-som-am57x_defconfig
+++ b/configs/cl-som-am57x_defconfig
@@ -33,6 +33,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ISO_PARTITION=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_SCSI_AHCI=y
 CONFIG_CMD_PCA953X=y
 CONFIG_LED_STATUS=y
 CONFIG_LED_STATUS_GPIO=y