one processor access multi ochip memory??

Haven't officially used a Nios for much more than hello world so far but from what I recall you just need to assign proper address ranges to instantiated memories (I think SOPC will also auto assign for you if you like, and generate header files with the address ranges as constants). A mutex is used when you have multiple processors hanging off the fabric trying to access a limited resource, such as a singular UART or SPI element or user instantiated module.