Kconfig: Make CMD_SPL_NAND_OFS only available when proper memory is used

This commit makes the CMD_SPL_NAND_OFS only visible when we use NAND
memory.
Before this change it was present when only CMD_SPL was enabled (and
would stay when board with other falcon boot medium is used).

Signed-off-by: Lukasz Majewski <lukma@denx.de>