Convert CONFIG_CMD_SPL_NAND_OFS to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SPL_NAND_OFS

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/devkit8000_defconfig b/configs/devkit8000_defconfig
index 71a382d..875c888 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -11,6 +11,7 @@
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
+CONFIG_CMD_SPL_NAND_OFS=0x680000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_NAND=y