MIPS: microMIPS: Add instruction formats.

Add structures for all the microMIPS instructions. Also add the
enumerations for all the bit fields for opcodes, functions, etc.

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: kevink@paralogos.com
Cc: ddaney.cavm@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/4921/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
(cherry picked from commit d7f19e43a4337d4d40ff5e241172912130d06a4c)