image: fix IH_ARCH_... values for uImage compatibility

Commit 555f45d8f9 ("image: Convert the IH_... values to enums")
accidentally changed some IH_ARCH_... values.

Prior to that commit, there existed a gap between IH_ARCH_M68K and
IH_ARCH_MICROBLAZE, like follows.

  #define IH_ARCH_SPARC64         11      /* Sparc 64 Bit */
  #define IH_ARCH_M68K            12      /* M68K         */
  #define IH_ARCH_MICROBLAZE      14      /* MicroBlaze   */
  #define IH_ARCH_NIOS2           15      /* Nios-II      */

The enum conversion broke the compatibility with existing uImage
files.  Reverting 555f45d8f9 will cause build error unfortunately,
so here is a more easy fix.

I dug the git history and figured out the gap was introduced by
commit 1117cbf2ad ("nios: remove nios-32 arch").  So, I revived
IH_ARCH_NIOS just for filling the gap.

I added comments to each enum block.  Once we assign a value to
IH_... it is not allowed to change it.

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -134,6 +134,9 @@ enum ih_category {
 
 /*
  * Operating System Codes
+ *
+ * The following are exposed to uImage header.
+ * Do not change values for backward compatibility.
  */
 enum {
 	IH_OS_INVALID		= 0,	/* Invalid OS	*/
@@ -167,6 +170,9 @@ enum {
 
 /*
  * CPU Architecture Codes (supported by Linux)
+ *
+ * The following are exposed to uImage header.
+ * Do not change values for backward compatibility.
  */
 enum {
 	IH_ARCH_INVALID		= 0,	/* Invalid CPU	*/
@@ -182,6 +188,7 @@ enum {
 	IH_ARCH_SPARC,			/* Sparc	*/
 	IH_ARCH_SPARC64,		/* Sparc 64 Bit */
 	IH_ARCH_M68K,			/* M68K		*/
+	IH_ARCH_NIOS,			/* Nios-32	*/
 	IH_ARCH_MICROBLAZE,		/* MicroBlaze   */
 	IH_ARCH_NIOS2,			/* Nios-II	*/
 	IH_ARCH_BLACKFIN,		/* Blackfin	*/
@@ -234,6 +241,9 @@ enum {
  *	U-Boot's command interpreter; this feature is especially
  *	useful when you configure U-Boot to use a real shell (hush)
  *	as command interpreter (=> Shell Scripts).
+ *
+ * The following are exposed to uImage header.
+ * Do not change values for backward compatibility.
  */
 
 enum {
@@ -273,6 +283,9 @@ enum {
 
 /*
  * Compression Types
+ *
+ * The following are exposed to uImage header.
+ * Do not change values for backward compatibility.
  */
 enum {
 	IH_COMP_NONE		= 0,	/*  No	 Compression Used	*/