you will need to create a text file that includes the missing informations.
<nios2_install>/bin/nios2-flash-override.txt and inside this file you include the informations like this example
--- Quote Start ---
[EPCS-010216]# EPCS64N (Spansion-lead-free) S25FL064A0LMFI001
sector_size = 65536
sector_count = 128
--- Quote End ---
The Spansion datasheet for this device says
Manufacture Identification 01h
Memory Type 02h
Memory Capacity 16h
putting all these informations together delivers 010216
so you will need to look into the datasheet of your device and find the require informations and created the file.
BTW: too bad that the nios forum has lost so many postings, there was a realy complete override file and i remember that epcs device was already listed ...