Convert CONFIG_SCSI to Kconfig

This converts the following to Kconfig:
   CONFIG_SCSI

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/A20-Olimex-SOM-EVB_defconfig b/configs/A20-Olimex-SOM-EVB_defconfig
index 6c87648..61fe5e6 100644
--- a/configs/A20-Olimex-SOM-EVB_defconfig
+++ b/configs/A20-Olimex-SOM-EVB_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_ISO_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set
+CONFIG_SCSI=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_RGMII=y
 CONFIG_SUN7I_GMAC=y