nios2: fix cached mode in clearing the BSS

As the generic board runs in cached mode, it should not use
"stwio" which bypass the cache.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Chin Liang See <clsee@altera.com>

arch/nios2/cpu/start.S

@@ -158,7 +158,7 @@ relocate_code:
 	ori	r6, r6, %lo(__bss_end)
 	beq	r5, r6, 5f
 
-4:	stwio	r0, 0(r5)
+4:	stw	r0, 0(r5)
 	addi	r5, r5, 4
 	bne	r5, r6, 4b
 5: