bootstage: Support relocating boostage data

Some boards cannot access pre-relocation data after relocation. Reserve
space for this and copy it during preparation for relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/bootstage.c

@@ -484,6 +484,11 @@ int bootstage_unstash(void *base, int size)
 	return 0;
 }
 
+int bootstage_get_size(void)
+{
+	return sizeof(struct bootstage_data);
+}
+
 int bootstage_init(bool first)
 {
 	struct bootstage_data *data;