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/start.S b/arch/riscv/cpu/start.S
index 3402d09..60ac8c6 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -111,8 +111,10 @@
 	bnez	tp, secondary_hart_loop
 #endif
 
+#ifdef CONFIG_OF_PRIOR_STAGE
 	la	t0, prior_stage_fdt_address
 	SREG	s1, 0(t0)
+#endif
 
 	jal	board_init_f_init_reserve