Some function had exceed 30 inline assembly register oprands limiation
when using LOONGSON2 version of MMI macros. We can avoid that by take
$at, which is register reserved for assembler, as temporary register.

As none of instructions used in these macros is pseudo, it is safe to
utilize $at here.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>