MIPS: uasm: Add u2u1 instruction builders

It will be used later one for the jalr and wsbh instructions.

[ralf@linux-mips.org: Dropped arch/mips/include/asm/uasm.h segment because
that was already added by 49e9529b9d43773307b8c73bd251b71784830c3d
[MIPS: uasm: add jalr instruction].

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/6724/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>