MIPS: uasm: Add u3u2u1 instruction builders

It will be used later on by the sllv and srlv instructions.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/6723/


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