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/ipam390_defconfig b/configs/ipam390_defconfig
index c97256d..702b311 100644
--- a/configs/ipam390_defconfig
+++ b/configs/ipam390_defconfig
@@ -18,6 +18,7 @@
 CONFIG_SYS_PROMPT="U-Boot > "
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
+CONFIG_CMD_SPL_NAND_OFS=0x00180000
 CONFIG_CMD_ASKENV=y
 CONFIG_CRC32_VERIFY=y
 # CONFIG_CMD_FLASH is not set