riscv: prior_stage_fdt_address should only be used when OF_PRIOR_STAGE is enabled

This patch will fix prior_stage_fdt_address write failure problem, when
AE350 boots from flash.

When AE350 boots from flash, prior_stage_fdt_address will be flash
address, we shall avoid it to be written.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
index 0cfd7d6..e9a8b43 100644
--- a/arch/riscv/cpu/cpu.c
+++ b/arch/riscv/cpu/cpu.c
@@ -15,7 +15,9 @@
  * The variables here must be stored in the data section since they are used
  * before the bss section is available.
  */
+#ifdef CONFIG_OF_PRIOR_STAGE
 phys_addr_t prior_stage_fdt_address __attribute__((section(".data")));
+#endif
 #ifndef CONFIG_XIP
 u32 hart_lottery __attribute__((section(".data"))) = 0;