fdtdec: make CONFIG_OF_PRIOR_STAGE available in SPL

The current preprocessor logic prevents CONFIG_OF_PRIOR_STAGE from being
used in U-Boot SPL. Change the logic to also make it available in U-Boot
SPL.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>