MIPS: uasm: Add u2u1 instruction builders

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

[[email protected]: 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 <[email protected]>
Cc: [email protected]
Patchwork: http://patchwork.linux-mips.org/patch/6724/
Signed-off-by: Ralf Baechle <[email protected]>