spl: Add a comment to spl_set_bd()

There is a strange feature to set global_data to a data-section variable
early in SPL. This only works if SPL actually has access to SRAM which is
not the case on x86, for eaxmple. Add a comment to this effect.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NSimon Glass <sjg@chromium.org>

common/spl/spl.c

@@ -378,6 +378,10 @@ static int spl_common_init(bool setup_malloc)
 
 void spl_set_bd(void)
 {
+	/*
+	 * NOTE: On some platforms (e.g. x86) bdata may be in flash and not
+	 * writeable.
+	 */
 	if (!gd->bd)
 		gd->bd = &bdata;
 }