MIPS: Add LDX and LWX instructions to uasm.

Needed by Octeon II optimized TLB handlers.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Pachwork: https://patchwork.linux-mips.org/patch/1903/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>