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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index afb6204..34a6975 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -23,6 +23,7 @@
 CONFIG_FASTBOOT_BUF_SIZE=0x07000000
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
+CONFIG_CMD_SPL_NAND_OFS=0x00080000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y
 # CONFIG_CMD_SETEXPR is not set