ata: Migrate CONFIG_SCSI_AHCI to Kconfig

And use 'imply' liberally.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/MPC8641HPCN_defconfig b/configs/MPC8641HPCN_defconfig
index dd2a92b..72ff192 100644
--- a/configs/MPC8641HPCN_defconfig
+++ b/configs/MPC8641HPCN_defconfig
@@ -15,6 +15,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT2=y
 CONFIG_DOS_PARTITION=y
+CONFIG_SCSI_AHCI=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_PHYLIB=y