image: fix IH_ARCH_... values for uImage compatibility

Commit 555f45d8 ("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 555f45d8 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 1117cbf2 ("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>