target-mips: Set Config5.XNP for R6 cores

Set Config5.XNP for R6 cores to indicate the extended LL/SC family
of instructions NOT present.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>