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/cairo_defconfig b/configs/cairo_defconfig
index 3122287..3045b83 100644
--- a/configs/cairo_defconfig
+++ b/configs/cairo_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
+CONFIG_CMD_SPL_NAND_OFS=0x240000
 CONFIG_CMD_ASKENV=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y