passage: Support an incoming passage

Plumb in the ability for U-Boot proper to accept an incoming standard
passage from a previous phase, such as SPL or TF-A. This allows data to
be passed from binary to binary when firmware is booting.

Series-changes: 2
- Rebase to master
- Rework global_data for new stdpass convention

Series-changes: 3
- Move passage.h into this patch
- Add passage_valid() to decide if stdpass was provided
- Move arch_passage_entry() into this patch
- Make the global_data fields present only when needed

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

lib/asm-offsets.c

@@ -48,5 +48,13 @@ int main(void)
 	DEFINE(GD_ENV_ADDR, offsetof(struct global_data, env_addr));
 #endif
 
+#if CONFIG_IS_ENABLED(PASSAGE_IN)
+	DEFINE(GD_PASSAGE_MACH, offsetof(struct global_data, passage_mach));
+	DEFINE(GD_PASSAGE_BLOBLIST,
+	       offsetof(struct global_data, passage_bloblist));
+	DEFINE(GD_PASSAGE_DTB,
+	       offsetof(struct global_data, passage_dtb));
+#endif
+
 	return 0;
 }