mips, uasm: Enable muhu opcode for MIPS R6

Enable the 'muhu' instruction, complementing the existing 'mulu', needed
to implement a MIPS32 BPF JIT.

Also fix a typo in the existing definition of 'dmulu'.

Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211005165408.2305108-2-johan.almbladh@anyfinetworks.com