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>
diff --git a/include/fdtdec.h b/include/fdtdec.h
index e6c22dd..635f530 100644
--- a/include/fdtdec.h
+++ b/include/fdtdec.h
@@ -54,7 +54,7 @@
 #define SPL_BUILD	0
 #endif
 
-#if CONFIG_IS_ENABLED(OF_PRIOR_STAGE)
+#ifdef CONFIG_OF_PRIOR_STAGE
 extern phys_addr_t prior_stage_fdt_address;
 #endif