memory map for SDRAM or ON_CHIP_RAM

Do you mean the memory map of your entire system so that you can find SDRAM or onchip, etc... When you compile a system library project in the Nios II IDE there will be a file called 'system.h' created which will have all the# define values you need to map your hardware. Use those# defines in your code so that if the memory mapping changes it will be updated automatically for you.

Now if you are asking about an internal memory map for each memory, that's up to you to decide (subdivide it any way you want to).