MIPS: uasm: Add support for LHU.

MIPS: uasm: Add support for LHU.

The follow-on BPF JIT patches use the LHU instruction, so add it.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Steven J. Hill <steven.hill@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15743/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>