MIPS: uasm: Add r6 MUL encoding

Add the R6 MUL instruction encoding for 3 operand signed multiply to
uasm so that KVM can use uasm for generating its entry point code at
runtime on R6.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>