spl: use panic_str instead of panic

For a simple static string, use panic_str() which prevents calling
printf needlessly.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>

common/spl/spl.c

@@ -452,7 +452,7 @@ ulong spl_relocate_stack_gd(void)
 #ifdef CONFIG_SPL_SYS_MALLOC_SIMPLE
 	if (CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN) {
 		if (!(gd->flags & GD_FLG_SPL_INIT))
-			panic("spl_init must be called before heap reloc");
+			panic_str("spl_init must be called before heap reloc");
 
 		ptr -= CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN;
 		gd->malloc_base = ptr;