intel/gen_decoder: Decode <group> inside <group>.

intel/gen_decoder: Decode <group> inside <group>.

Now we can decode a <group> tag inside another <group> tag, and properly
print its indices and content.

v2: Use push/pop stack to fields, groups and iters (Lionel).
v3: Add assert(iter->level < DECODE_MAX_ARRAY_DEPTH) (Lionel).

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>