MIPS: ieee754.h: Supplement comments for special values

Add the remaining missing comments for IEEE 754 special value array
indices.  Reindent macro definitions for consistency.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9671/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>