ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/db-88f6820-gp_defconfig b/configs/db-88f6820-gp_defconfig
index beac266..5cc2781 100644
--- a/configs/db-88f6820-gp_defconfig
+++ b/configs/db-88f6820-gp_defconfig
@@ -38,6 +38,7 @@
 # CONFIG_SPL_PARTITION_UUIDS is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SPL_OF_TRANSLATE=y
+CONFIG_SCSI_AHCI=y
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_SDMA=y
 CONFIG_MMC_SDHCI_MV=y